I think there was a thread on this a couple years back with some very informative ideas. Not time this morning for me to search for it. Lately I've been wearing a vest - (same as a waistcoat ??) because of the pocket thing - picks, capo, tuner - and a fairly dressy shirt and pants depending on the situation.

I've noticed that bluegrass bands in my area (NW USA)either wear "suits and hats" ala Bill Monroe, or are really casual - sloppy jeans, hung out T-shirts, sneakers, etc. I prefer the "suited" look. Folk groups (the very few there are) fall somewhere in between.