Bromoflexual saidMossimo Athletic Fit T-shirts. $8 each at Target, and the brahs over at bodybuilding.com are all over them.
The Mossimo shirts really are a hidden bargain, and do fit well. The fabric is light and drapes well. They're not quite as brutally tailored as the Abercrombie "muscle" cut shirts or the Hollister "surfer" cut shirts, but the thinner fabric makes up for that. Abercrombie and Hollister shirts tend to be too heavy to wear in really hot weather.
I also like the Ralph Lauren custom fit shirts. I'm 5'9", 160 lbs. mesomorph (not big by any stretch), and my frame really shows off well in a small RL shirt. For casual t-shirts, I almost always buy a small, and get mediums for work shirts and less casual wear.
The Express 1MX cut is also pretty brutal, particularly in the ultra slim lines. I have a couple of those shirts, and I have to really be feeling pretty to wear them.
If you're looking for some quick bargains, TJ Maxx and Marshalls are everyone's dirty little secret, and if you keep up with their inventory turnover, you can get most major brands for dirt cheap.