I use a simple visual aid (on a 3 x 5 file card) showing 6 basic chords for each key (7 for blues):

A ... F#m ... D ... Bm . B ... E ... C#m ... A
C ... Am ... F ... Dm . D ... G ... Em ... C
D ... Bm ... G ... Em . E ... A ... F#m ... D
E ... C#m ... A ... F#m . F# ... B ... G#m ... E
F ... Dm ... Bb ... Gm . G ... C ... Am ... F
G ... Em ... C ... Am . A ... D ... Bm ... G

With that on the table beside my song book, I find it pretty easy to work out a rough system of chords without any analytical thinking. Then I tweak it by experimenting with variations on the 6 basic chords:
- playing the chord higher on the fingerboard;
- omitting the high E and sometimes the B string;
- changing one note in the chord by adding, removing, or moving a finger, usually on the G, B, or E string.

Changing a note makes it a different chord, but I don't know the names of all those other chords. So I just write the variant note as a lower-case letter after the name of the basic chord. Except that if the changed note makes it into a 7th chord, sometimes I'll write 7 instead.

Extremely rarely, I find that none of these variations works and I have to try every possible combination of notes to find a chord that works. The result is usually a chord I can't find in any chord book anyway, so I just diagram it at the bottom of the page and refer to it as X.