This is a good topic (chord substitution) and something I'm always trying to brush up on. I have a few "stock" formula type things that work well for intros and endings. Here's one I was working on this morning for the tune "shawdow of your smile".

Instead of ending right on the "I" chord the following chromatic ending sounds really nice:

bV IV III bIII II bII I

All the chords are dominant (sounds nice with altered notes like sharp 9's,13th's, sharp 11's...)

So in the key of G this would be: Db7, C7, B7, A7, Abmaj9, G maj



I learned some real nice blues progression that break away from the I, IV, V chords. If anybody is interested I could try to a copy.