As an occasional nerd, I’ve learned a fashion tip over the years that would have served me well had I learned it earlier in life.
Wear clothes that fit.
It may seem obvious, but it wasn’t to me. When something is too small, it's obvious - it's tight, uncomfortable, or you can’t even fit into it. Too big, though, is something else all together. Technology conference t-shirts tend to come in two sizes, Extra-Large and Whatever-Is-Left.
In high school, my pants were too big and my t-shirts hung over my frail frame like bed sheets (as was the custom at the time).
It turns out that if you take a few minutes to find the size that fits you comfortably, you will feel better and look better. Try this: find the smallest size that you can fit into and then get one size larger.
While this advice is primarily for nerds, it also applies to most geeks and some dorks.