Conventions are a great place to get some additional publicity for your comic. There are a number of ways to use the convention to get more viewers for your comic, and most of them will do wonders for your comic. Just remember to have some fun with them and you should do well.
Big Jar of Toys: Take some cheap promotional toys and fill a jar with them. Make sure that your webcomic and its URL are on them first, obviously, and make sure that the item is sort of fun and applies to your comic. The jar will empty by the end of your time at the convention, and all of a sudden a lot of people will have your webcomic’s URL in their pocket.
Cosplay: It may sound silly, but sometimes silly works. The idea here is simple: People take pictures of other people in outfits. If yours is interesting enough, you will also get pictured. If it is really interesting, they will find out what the costume is of and post contact information about what they find. Ergo, if you dress up you have a chance of making someone’s wall and getting some fame for your webcomic.
Pamphlets: Paper may sound outdated, but if you offer a pamphlets that summarizes your webcomic, lists important characters and story arcs, and offers trivia and contact information, you create a handy-dandy reference for your comic. As this can help people get into your webcomic by making it less intimidating, especially if your webcomic has been around for any amount of time, you also create a ticket into enjoying your comic.
Sponsor Passes: Conventions usually offer a sponsor pass; it is a little more expensive, but offers a small ad space on the convention schedule itself. This means that, for paying a little extra, you get your URL and a cute ad in the hands of everyone at the convention, at least those that are supposed to be there. Sometimes this also means becoming part of the links on the site, which is worth it as well; for those doing even basic SEO for the webcomic, this means a backlink to your site that actually increases your site’s relevancy, helping your ranking on search engines.
Table Contest: You can also run a convention from your table. This means that you offer a decent prize, run a lottery, and the winner(s) walk away with a decent prize. Just make sure that you don’t run the drawing at the same time as any major events and that local laws allow it, and you should get some decent publicity for your webcomic.
This is the tip of the iceberg. You can show up on panels and help out the convention, but you should explore opportunities yourself. Have fun, and you should have better business after the convention.