my rules for casual fashion:
the first thing you can do is don't wear baggy t-shirts. nothing looks worse than a baggy t shirt. in fact, the tighter the better (up to a point). also, 60/40 blends look much better than 100% cotton shirts, in terms of movement and showing your body and stuff. only problem is you're much more likely to stink in a blend shirt, so don't forget that deoderant.
next thing is shoes. unfortuantely, although nikes and reeboks are uber-comfortable, they look like ass and always have. get some chuck taylors, or puma/addidas soccer style shoes. they key with shoes is to avoid bulk and plastic. otherwise you walk around looking like the kids in the future in Bill and Ted's Bogus Journey. use gel inserts to avoid beating your feet to death in those crappy shoes.
lastly, pants. you're doing ok here. jeans are still cool. if you're thin, 501's are good. if you're less than thin, 550's. also, slacks look great with a fitting t-shirt and the above shoes. cargo pants were only cool 4 years ago, stay away from those, besides, they go against our "no bulk" mantra.
for outer wear, a zip-up sweatshirt rocks and is cheap. again, baggy is not allowed. peacoats are cool, and they haven't been too overexposed yet. these can be bigger, since they are cut that way to begin with. jeans jackets look cool when you're standing up but when you sit down, you'll expose plumber's crack to the whole world.
for shirts, you can always find neato blended shirts at thrift stores, but not everyone likes that look. a good alternative is jockey's t-shirt undershirts. they look great as a normal old shirt, fit nice and stuff.
for hats, baseball hats are cool if you wear them like a normal person and keep the bill curved to match the width of your head. beanies are cool. or try something off the wall, like a bowler.
oh for socks, plain white socks are fine, but also get some black socks. black socks can make a big difference in the look, usually for the better.
yay.