1. The simplest way to improvise over a II-V-I in major is to play the major scale of the I chord.
2. The simplest way to improvise over a II-V-I in minor is to play the harmonic minor scale of the I chord.
3. The simplest way to improvise over a blues progression is to play the minor blues scale of the key.
4. A V chord in major may be satisfied with any dominant 7th scale except minor blues.
5. A V chord in minor should be treated with the 5th mode of the harmonic minor or a Super Locrian scale. These scales supply alterations that are consistent with the minor key signature and make the resolution sound natural.
6. A bII chord in minor sounds best with a lydian, b7 scale which is the same sound as a Super Locrian on the V chord. Lydian, b7 also sounds good on a bII chord in major.
7. A IV7 in major sounds best with a lydian, b7 scale. It supplies the #11 which is in the key signature.
8. A II7 in major often sounds good with lydian, b7 because the 2nd and 6th (9th and 13th) are consistent with the key.
9. A minor 7th a 5th above a bass note is a dom7sus4 (like II and V at the same time). Example: Dmi7/G
10. If a note sounding in one chord can continue into the next chord, it's generally a good idea.