Nicer clothes. This one depends a lot on how style-conscious you and your usual environs are, of course, and probably a lot of people here don't need to be told this. On the other hand, I'm sure there are also people here like I was a few years ago, when I not only didn't care about style or fashion, I also basically just wore whatever was most cheap and comfortable in any given scenario. That was a mistake.

I found that having some respectably-nice-but-not-too-formal clothes can be a big plus in an environment where everybody expects a t-shirt with jeans/shorts. Think collared button-down shirt and slacks, with some nicer-than-everyday shoes. You don't need many sets of such clothes - the idea is to have something nice to wear when you expect it to be noticed, not necessarily to overhaul your whole wardrobe - but for things like a party that a friend is throwing where you know there will be people who you haven't met, it's worth the investment. This is especially true if you're looking for a new relationship.

It may take some work, if you're not used to it, to figure out the right balance of formality and style. That's OK. In my social circles it is often very easy to be the best-dressed guy in the room. A suit is too much. A coat and tie is too much. A $30 shirt and a $40 pair of slacks, plus some nice-ish shoes, will go a long way.