I've been wearing them under pretty much everything, but I'm especially fond of the look under sheer blouses. The lace peeks out a slight bit, and when the light hits just right you can see the faint outline. Sexy in a subtle sort of way...
[the only problem with my girly room? it's almost too conducive for playing dress-up... my sister came over & we ended up with the entire contents of my closet on my bed.]
I find they work well under camisoles too, sparing you the trouble of an unintentionally exposed bra...
p.s. similarly, my rules for exposed bras here...
[all tops: CAbi, pink shorts: forever 21, all bandeaus: american eagle]