I’ve listed some guidelines to dominant chords that beginners and musicians coming from other types of music often need to have explained to them, from the mild to the spicy.  They are not ‘rules’ but ‘guidelines’ that will help open up an understanding of what jazz players have been doing with dominants for well over half a century.

For the sake of argument, we'll talk about the key of D (two sharps), in which A7 is the V chord.

1. The essential tones of a dominant are the Major 3 and flat 7 -- C# and G for an A7. Those are the only tones an accompanist really 'needs' to play to express the V chord. Also, these are the strongest notes to target in a dominant phrase to explicitly state the V chord in the harmony.
2. A 'functioning' dominant is one that is going to its I or i chord. A non-functioning dominant would be like the Bb7 in a Bb blues, because is not the V of any chord (until it goes to the IV chord).  Another example is a II7 that eventually changes to a ii-7 before moving to the V and back to the I.
3. The dominant in a ii-V is a functioning dominant. In fast tune, you are likely to just think of the V chord instead of the ii and then the V for the cadence, especially since the ii and V are diatonic. If you target the b7 and the root of the V early in your ii-V line, you have already expressed the 'ii' via 'essential tones' (essential tones are 3rds and 7ths).
4. Any functioning Dom.7 chord can be altered with almost any type of 'extension.' The only taboo note is the Major 7 -- you wouldn't want to play an Ab in an A7 unless it was a chromatic movement (see 'bebop' scale). Traditionally, the 11 is somewhat 'weak' also. Adding a b9, 9, #9, b5, #5, 13 are all cool to add in your A7, however if you have one player thinking A9 and one player thinking A7b9, you might get some bad 'rub' if they aren't listening to each other.  The ‘outside’ notes can help ‘lead’ the listener chromatically back to the I, which where a functioning dominant goes.
5. While A Mixolydian is the 'diatonic' way to play over our A7, you can also play a BbminMaj7 scale, or a DminMaj7 scale. These are referred to as the 'altered' scale, they are just different modes of the Harmonic or Melodic Minor when you start on the A. They bring out various altered tones as listed in guideline #4.
6. You can also play a Bbdim arpeggio/chord or any inversion of it (Gdim, C#/Dbdim, Edim). This is like an A7b9 substitution.
7. You can substitute the tritone Dominant of a functioning V instead of the V7. Instead of A7, you can substitute Eb7. A tritone sub voicing is also a V7#11 voicing.
8. You can substitute the dominant rooted at any note of the V's tritone diminished arpeggio on a functioning V chord. The tritone sub in D is Eb7 (Eb is the tritone of the V, which is A). The Ebdim arpeggio is Eb, A, C, Gb. For the A7 five chord, you can sub Eb7, C7, Gb7. As stated in guideline #7, Eb is a A7#11 voicing. The C7 is A7(b9,#9). The Gb7 is A7(b9,13).
9. Because of #6 and #8, you can play the A Half-Whole diminished scale for a A7 (A, A#, C, C#, D#, E, F#, G). If it is easier, you can think of this scale as E Whole-Half scale (E being the ii chord).
10. Pretty much any diatonic chord can be re-harmonized as a dominant. The vi chord can be played as a VI7 (A Train). The ii chord can be played as a II7 (There is No Greater Love). The iii chord can be played as III7 (All of Me). I can't think of a vii chord going dominant, but the bVII7 dominant is the 'back-door' dominant that can resolve to the I (G-7 / C7 / Dmaj7). Remember C7 is one of the 4 dominants out of the tritone diminished in guideline #8.
11. You can play the whole tone scale on Dom7#11 or Dom7#5 chords (A whole-tone on A7). It can sound pretty 'out' if the rest of the band isn't hip and seems to work better in some scenarios.
12. The melodic minor has 2 diatonic dominants. The MM IV is Dom7#11, the MM V is Dom7(b13). For playing Bb melodic minor over A7, you can try Eb7#11 or F7b13. If you were thinking E Harmonic minor, you could try A7#11 or B7b13.
13. The Barry Harris 'Six to Diminished' concept re-harmonizes the diatonic chords to either be a 'Sixth' chord or Dom7b9. The 'V' chords are played using ‘diminished’ voicings with the V's b9 as in guideline #6.  The diminished voicing of the V always leads to the neighboring inversion of the I6 voicing.  There is an extra chord inserted to account for the #5 of the 'Barry Harris' scale explained later. For D major, the chords are all the B-6 inversions (which serve as the I, III, V, VI chords), plus all the C#dim inversions (which serve as the II, IV, bVI and VII chords). So in D Major: I = D6, II = Edim (A7b9), III = F#-6 or F#maj6 (don't voice 3rd – and inverted I6), IV = Gdim (A7b9), V= A9/11/13 (inverted I6), bVI = A#dim or Bbdim (A7b9), VI = B-7 (inverted I6), VII = C#dim (A7b9). This creates a chord scale that is constantly descending from V (A7b9) to I (inversion of D6). It diatonically relates to the 3rd mode of the Harmonic Minor Scale (minus the 5). So, the 'Barry Harris' Maj#5 scale is the VI as Harmonic Minor. For D, the Maj#5 Barry Harris scale is essentially B Harmonic Minor (plus the A note). This scale has all the B-6 and C#dim inversions listed above.
    

The Altered scale is the seventh mode of the Melodic Minor (aka Jazz-Minor) scale.  It is a great scale for finding leading tones over a "functioning" V in a ii-V-I.  These leading tones are 'outside' the Key center of the ii-V-I, which create even more tension over the already tension-inducing V.  This tension of the V is resolved when you get back to the I.

The typical way to play the Altered scale (aka Super-Locrian), is to sight the root of the Altered scale on the root of the dominant.  Alternatively, you can sight the Melodic Minor's root on the b9 of the V chord -- one semi-tone up from the root of the V.

The Super-Locrian/Altered scale is really the 7th mode of the Melodic Minor -- the root of the Altered scale is the 7th degree of the Melodic Minor scale.  Additionally, there are other modes of the Melodic Minor that work nicely over dominant chords in other situations.  

The Lydian b7 (4th mode of Melodic Minor) works great over Dominants with a #11.  If you are familiar sighting the altered scale, you can sight the root of the altered scale on the #11 of the Dominant. Or you can sight the root of the Jazz Minor on the 5 of the V.

For a Dominant with a #5, you can play the Mixolydian b6 - the fifth mode of the Melodic Minor.  Sight the start of the altered on the 3rd of the V chord you are playing over.  Otherwise, sight the Melodic Minor starting on the 4.

Finally, a Dorian b2 (3rd mode of the Melodic Minor) works over a Dominant with a #9 or a b9.  Sight the altered scale starting at the V chord's 13.  Or sight the Melodic Minor on the b7.

I learned the Melodic Minor via the Altered Scale, so I tend to sight with the Major 7 of the Melodic Minor.  But it is really best to sight the Melodic Minor.  When you first learn the Altered scale, your melodic lines will probably sound quite foreign if you think of the 7 of the Melodic minor as the root -- similar to when you attempt to play the Locrian mode out of context.  The 7's of the scale are really pulling the westerner's ear toward the next tone up, which is the root of the key.

If you play lines with the Melodic Minor in mind, you will still be creating tension on the V with the outside note, but the lines will sound a little more familiar and groovy somehow.  The best way to inflect the Melodic Minor sound is to focus on notes in the Melodic Minor arpeggio 1, b3, 5, 7.  This isn't the end approach to take with the substitute modes, but it worth starting with.  By focusing on the Melodic Minor arpeggio, you will begin to see the other modes in relationship to the Melodic Minor harmony.

The chart provided shows all the Super-Locrian, Lydian b7, Mixolydian b6 and Dorian b9 modes against the Mixolydian of a V chord.

In addition, I've broken each of my mode boxes out and repeated the information for each neck position.  The name 'r6-vi' Aeolian Box is my description of the 4-5 fret position where the Aeolian Mode for would start with its root on the 6th guitar string (low E).  That is the shape of the notes in the position, but the V is still the root in this context.

With the mode boxes, I've also stripped this box down to the arpeggios for each mode.  The yellow ring indicates the Root of the Melodic Minor.  Therefore, for the right-hand, stripped down box of the Super-Lociran, Lydian b7, etc., shows the Melodic Minor arpeggio, with the yellow ring as the root.  This outlines how to play a really strong Jazz-Minor sound against these altered dominants.

Try looping a passage with an altered dominant like a Dom #11, and find the mode of the Melodic Minor (Lydian b7 in this case) that works best over it.  Play the melodic minor mode over the V chord -- maybe even imagine the arpeggio of the V chord dimly sitting 'underneath' the mode you are playing.  Then try and make a line solely comprised of the Melodic Minor arpeggio before resolving to the next chord.  Try this in various positions over the neck until you start to see the juxtaposed relationship.  You should practice until you can see your Mixolydian notes or your V arpeggio, and then immediately super-impose the applicable Melodic Minor mode, understanding the 'essential tones' (3rd & 7th) of the substitute mode.

Once you are facile at seeing each mode in relation to the Dominant arpeggio the V, you can begin focusing on structuring lines that really capture the essential tones of the V chord while also including the altered notes of the chord on the lead sheet, pulling you toward resolution with an essential tone of the one.

I made this chart to begin creating visual "anchors" to the diatonic modes I am already familiar with.

I've been practicing the Harmonic Minor, which I know serves as the Altered Scale when its root is on the b9 of a Dominant.

Now I need "anchor" it to the modes I already am familiar with, so switching to the Altered Scale will be instantaneous.  I'm practicing switching between each section of the Altered scale with the corresponding mode in the diagram, measure to measure.  Then I'm practicing a ii - V7alt - I progression to get to the scale in time.

I basically visualize 5 "boxes" which correspond to five modes where the root is on the 6th string.  The Lydian and Locrian are so close to the Phrygian and Ionian respectively that I just picture the later boxes.

I use 'r6 Aeolian' to connote the shape of the Aeolian mode where it starts on the sixth string.  I'm using 'Vr4' to connote the root, arpeggio or scale for the V (Dominant) of the diatonic progression where it appears on the fourth string.