The trick is to find the nearest "easy" key to play in below the original key. Count the difference in semitones and place the capo on that fret.
Easy keys, at least for me (I-IV-V-vi):
- G-C-D-Em
- D-G-A-Bm
- A-D-E-F#m
- C-F-G-Am
- E-A-B-C#m
For example, you mentioned playing in Eb. The key of D (with its IV and V chords G and A respectively) is one semitone below, so put the capo on the 1st fret and play in D.
Here's a table if you like a quasi-rule to follow. Some keys can be played in multiple capo settings; my preferences are listed in order. And this is not exhaustive - you can certainly capo much higher if you prefer, but you will lose any bassy-ness of the guitar in the process, if that's your thing.
Concert Key Capo Keys-Fret #s
A G-2, A-0
A#/Bb G-3, A-1
B A-2, G-4
C C-0, G-5
C#/Db C-1
D D-0, C-2
D#/Eb D-1, C-3
E D-2, E-0
F D-3, E-1
F#/Gb E-2, D-4
G G-0, D-5
G#/Ab G-1