Thanks for all the replies. I checked my two new suits and found that one of them is 100% wool. It creases slightly when I sit, and the creases come out by themselves after it's been hung for a while, so that's not so bad. The other one that caught my attention for creasing really badly is part linen/part rayon. Ah, no wonder. Too bad because it's really cute.
I'll add the "scrunch test" to my list of things I look at when choosing clothes in the store. Thanks, great tip! I'll also try to find the fabric content (they sure like to hide it these days) and buy only wools if I can. The wool suit I have is fairly lightweight so if I can find more like it that will be great.
Hey, while I'm here.... has anybody had luck using Woolite at home to clean clothing marked "dry clean only"? Does that work, or is it a good way to ruin expensive clothes?