You don't need to wear something very tight, just a tad. The day before bring a nice shirt, and put it in your cubicle. Wear a wife beater under shirt you'll be wearing that day. Spill a bit of coffee in your shirt, walk around a bit and change in your cubicle since you have a shirt there.
Seriously though, most girls can tell if your big through dress clothes. At least in comparison to the average twink. Do what Go For It said, invite the girls you want to hit up for drinks. Make it seem like its just what normal people do when they're tired from a hard day at work (it is). Or even invite everyone on the floor. Break the ice, and go from there. Make it the Friday event after work that way you guys will hang out quite a bit.
A word of caution don't pretend you want a relationship at the work place or they will make your life a living hell. Be honest on what you want from them. So things can be cool, and maybe you have a good friend with benefits at work.