A couple of things Lyda didn't mention in the (essentially) free section:
Update:I posted this simultaneously with Lyda's post above, and she responds to my convention note in comments on this one.
Conventions. If you've got a small press credit or a professional short story credit (or something bigger) the chances are pretty good that your nearest available convention will let you come for free if you agree to be on some minimum number of panels, usually three. If you're moderately articulate and have opinions on writing and a few fannish passions (Star Trek, LOTR, Buffy, Heinlein, whatever) you can do your career a lot of good this way. You can also have a ball and make new friends. This one is variable cost-since it depends on how far from the con you happen to live and whether you can do the thing from home, but can be quite cheap, or even free.
Writing reviews. There are a lot of F&SF fanzines, semi-pro, and even pro venues that are looking for reviewers of F&SF books and other media. This one's often a twofer, free press and free stuff, because many of the higher profile venues will actually send you books to review.
Of course, these things do cost time, and even if you're a devout believer in the power of self-promotion, you should have some calculation for what your time is worth vs. expected return.