Button down denim shirts are just impossibly chic right now. Wear them with jeans for that whole denim on denim look. Or wear it open over a tank! Buy this Theory one at Neiman Marcus for $170. It looks so soft!
If you aren't ready to commit so much money to a trend you're only trying out, you can get this one at Express for $60. I like the darker color, but not as sold on two pockets - very Western.