​Frank Gambale offered this exercise for playing over chord changes on the 'No Guitar Is Safe' podcast.

Mr. Gambale uses the same strategy so many others use to play over chord changes, which -- in the music he plays -- can come as much as four times in a bar.  The secret is targeting one or two notes in an arpeggio -- essential tones.  The 3rd and 7th intervals are the ones that immediately state the quality of the chord, so that is the obviously place to start.  But being able to get to any interval will help you see the fretboard in a way that lets you play effortlessly over changes.

The Exercise:
Start simple -- looping two bars of E-minor and two bars of G-minor. Play the root 'E' first beat of Emin and Play 'G' on the first beat of G-minor.  Slowly work in the accompanying dorian scale as you feel ready.

From there, target the third of each chord.  Count up the dorian scale tones to the 3rd note from the E for Emin and the 3rd scale tone up from G on Gmin (Hint: you now play G at the start of Emin and Bb at the start of Gmin). Play this note on the first beat of each new chord. Make sure you are finding the equivalent note everywhere in a position, and eventually everywhere on the neck.  The 3rd of a minor is flatted -- it's one fret down from where it appears if the chord were major.

Next, target the flat-7th for each chord.  Loop the four bars and play the 7th on the first beat of each chord, adding notes after it.

From there you can move on to the five, and eventually all the tones, the 9, the 13, the 11. 

If you want to get fancy, target the tone for a specific beat of the chord, or try targeting two non-adjacent tones in a row.

Do the exercise moving two different chords -- Major to Minor, The V Dominant of a Major to the Major, etc.  Find the tones you like for each chord.  Find the tones everywhere on the neck.  After all that has been mastered, find the tones that are 'outside' the scale and know what they are.

Every time you get stuck on a passage of a tune, like a turnaround, break out that chord change and loop it.  Stretch it out and start slow as you can.  Target the 3rd and 7th of each chord.  Next, target all the arpeggio tones, then all the scale tones, finally all the 'altered' tones.  Make sure you are practicing in every position or visualization you have on the neck.

Your voice is the first melodic instrument you probably ever learned, and is the most accessible to the breadth of your audience.  I would argue the more a lead instrument sounds like someone singing, the broader appeal it will have.

I am not a singer and hate singing, BUT -- there are a number of exercises involving singing that will still help with jazz guitar.  Some are more of a time investment than others, where you may feel it takes away from practicing more 'practical' things, but remember that singing is completely portable.  Even just singing around the house washing dishes will keep you creatively developing melodic 'muscles' in your brain.

1) Sing when nobody is around the house or in your car.
Just coming up with melodies when you don't have your guitar is like calisthenics for the melody muscles in your brain.  Imagine a chord or a bar in a standard you are learning and sing a line over it, or come up with a melody and imagine an arpeggiated chord behind it.  Even if you have no idea what chords or notes you are singing, you are still connecting with the art and vibrations of improvisational music.  Or just sing what comes in your brain -- your are still crossing that important bridge from brain to muscle.

2) Sing along to music while it is playing.
Sing along to jazz or even pop music, even if you are just improvising over the harmony.  Try repeating phrases you hear in a solo on the fly.  I like to do this driving to a session -- it's like my first warm-up.  It makes my ear more more acute to what is going on. It exercises your listening muscles.  Listening is as important as anything else playing jazz and singing back a line can give you instant feedback on how acutely you are listening.

3) Sing when you are playing a solo.
Try to sing along with your solo.  This obviously helps with one of singing's biggest rewards: phrasing.  It's not difficult to play sixteenth notes for 32 bars straight without a rest and the fact that you cannot sing along to such a solo should tell you something.  Rests define melody as much as harmonic choices -- they are the 'negative space' that pop the melody out and encapsulate emphasis.  Jazz, not to mention music as a whole, was largely developed by instruments which require human breath.  Musical phrases often 'work' precisely because they emulate phrases of speech from a particular language.  Stewart Copeland likes to explain that even the most complex 12/8 rhythms from the farthest reaches of the world are really just templates of phrases in the local dialect for "I went down to the corner store to buy some milk" or something similar.  Even if it feels dumb, even if you are not playing the same note you singing, even if you are just grunting, even if you have never tried it -- try singing to your solos!  You will be amazed at how your timing suddenly becomes more direct and fluid.  Even non-melodic grunts will give your solo a 'conversational' reference point. 

4) Play a chord, sing a line, play the chord again, play the line.
Even if you know you are going to stumble, do it. It exercises all your jazz muscles.  It will take you out of the visual strategies you've built up eyeing where your fingers should go, what notes go with what chords, etc.  This is something I saw Barney Kessel teach in a video.  It helps build a personal vocabulary.  It gives you lines with 'real' phrasing.  It gives you a fresh and even personal perspective on improvisation over a harmony.

5) Play a note, sing a note, sing an interval of the note, play the interval.
This helps train your ears.  Even if you are getting in the ballpark, you are working your ear, but pay attention to your progress.

6) Transcribe a solo and be able to sing every note.
Transcribing from a master is one of the most beneficial things you can do to help learn the language of jazz.  This helps you really, really learn a solo.  Transcribing it to your voice puts it in your body, not just your fingers.  This exercise will also help you with your phrasing when you play it on guitar.  This is an extreme exercise, but you will get results from it.  When you are able to sing a solo, that solo is part of you, just like all those advertisement jingles and TV themes from your childhood.

You have probably heard someone talk about these exercises before so consider this is just a "blue-moon" reminder.  It will wake up something very deep inside your improvisational ability.  If you are like me, one reason you are playing instrumental guitar music is precisely because you *can't* sing (or rather don't sing well).  Even if you can't sing, don't sing or won't sing, singing will still improve your guitar playing.

To outline chord changes during a solo, your best bet is to target the chord's 'guide tones.'  Guide tones are the key tones that differentiate the chords.  Every chord has a root -- so that can't be a guide tone.  Unless you are playing half-diminished or altered voicing, a chord will generally have a natural fifth, so that doesn't outline the basic chord type either.

Thirds and sevenths are the best tones to hit in your passage over a given chord to outline it's harmony.  From those two tones, you can instantly distinguish between Major/Minor/Dominant.  Minor thirds immediately highlight the minor sound and the flat seventh will immediately signal 'this is not a major chord.'

Once you know how to introduce a phrase with a flat third over a minor chord and a flat seven over a dominant, your listeners will instinctively hear the chord progression just in your solo phrases alone.

Eventually you should take a song and, for each bar,  start your phrase with the appropriate third or seventh for that bar's chord.

It is key that you start with 'guide tones' as your target tones, but eventually you will want to break out of the habit of always grabbing those notes first to start a phrase over a chord.

Once you are ready to expand beyond the guide tones, try this exercise:

Pick out another scale tone and map it over the chord passage.  For instance -- take a ii-V-I and start the measure with the ninth of each chord.  For a four bar ii-V-I in C major, you will start bar 1's phrase with an E (the ninth of D-7). On bar 2, start a phrase with an A (ninth of G7), and finally start your bar 3 phrase with a D (ninth of Cmaj7).

Once you are comfortable with the ii-V-I, grab a standard you know fairly well and go through the whole tune starting each phrase with the ninth and then continuing to improvise with the scale or arpeggio you've chosen for that chord.

The next 'target tone' to start each phrase with may be a thirteenth.  Map out what the thirteenth is for each chord and play that note to start your phrase when that chord comes along.  Start with a simple set of changes and then move on to a song. Then maybe to a song with more complex harmonic modulations.

After that, move on to all the other tones.  What if you play a b3 to start every chord -- even the dominants and major chords?  Can you make it sound musical?  Can you make it make sense? How about a b5/#11?

Once you are confident of finding a certain interval for any chord in a progression, practice 'leading in' to that target tone from the preceeding chord's notes/scale -- especially when the tune 'hangs' on a chord for more than one bar. 

For instance, in your looped four-bar ii-V-I in C major ( D-7 / G7 / Cmaj7 / Cmaj ):

On the last bar, take a rest on the first half of the measure and then resume soloing in your C major arpeggio leading into your target tone for D minor and continue in your D minor arpeggio or scale without stopping.  

Some tunes to try this on for the starkest effect might be the modulation to the B section of 'So What', going into the fifth bar of 'Green Dolphin Street', going into the half-step modulation for the B section of 'Girl From Impanema', the third bar of 'Solar,' etc.  The goal is to move to these target tones as fluidly as possible, even from an 'alien' harmony. 

​Once you know where all the intervals are for a chord, it will be harder to get lost.  It's like when you are walking in a city and every turn you make, you see a building you recognize, no matter what neighborhood you venture to.  Even if you wake up and get off a bus or pop up from a new subway station -- as soon as you see that landmark, you can quickly navigate to anywhere you want to go. 

This exercise will not only help you identify the intervals across your fretboard for any chord harmony, but it will also reinforce that interval's sound against the chord's harmony. For instance, starting with a 13th (same as sixth) on a Major sound will briefly cue your ear into that chord's relative minor.  You will probably find certain intervals you gravitate towards for different chords and passages.