My best bet has been mens jeans. I'm in Levi's 541's Slim Straight jeans and I'm a happy camper. I wear a size 34/30 and the jeans sit 2 or so inches below my belly button.
+1 - men's. (in fact, 34/32 - same size!) the waste can gap a little, but they don't fall down.
I wear men's shirts, too - my big shoulders/arms fit better.
Then again, someone at work nominated me for that show "what not to wear" (honestly, I'm not making that up).
My photoblog http://dragons-fly-peacefully.blogspot.com/
Bacchetta Giro (recumbent commuter)
Bacchetta Corsa (recumbent "fast" bike)
Greespeed X3 (recumbent "just for fun" trike)
Strada Velomobile
I will never buy another bike!