There are two major categories when it comes to promoting your comic.
Free Self-Promotion.
Enter contests or art jams.
Webcomic Idol. Global Comic Jam. Local 24-Hour Comic Day events. The jam forums on various message boards.
Forum posts and signatures.
Take a panel or pin-up from your comic, superimpose the title and a short, catchy description (Like "Heliothaumic: Sci-Fi Elves!" or "Dead Winter: "Not Your Typical Zombie Comic!" ), make it banner-sized, and post it as your sig image on any forum that allows sigs. If they don't allow sig images, post a text link.
Art archive accounts.
A Deviant Art account, updated simultaneously with your comic's main site, might get you more readers. Some people are very loyal to community sites like DA, and would rather read a comic there than go offsite for it.
Guest strips.
If a comic you like that has a substantial readership is about to take a break or go on hiatus, volunteer to do a guest strip for it. Most comics that get guest strips will repay the contributor with a link back to their site.
Toplists / Ranking sites.
The Webcomics List. Comicrank. Webbed Comics. Comic Nation. There are tons of voting, listing, and ranking sites, and most have a policy of accepting all submissions. You'll have to do the work to get the votes, but the reward is a prominently displayed banner that drives traffic to your comic.
StumbleUpon, Reddit, etc.
Ideal, if your comic is a humor or strip comic with topical references. Stick Reddit, Digg, and StumbleUpon buttons on your site, and ask readers to click them.
Link lists.
Piperka, The Belfry, KillBoredom, ComicAlert, OnlineComics.net.
LJ community posts.
Join communities like webcomicscenter, webcomics, cartoonists, altcomix, and comics, and promote yourself whenever you have a new comic, storyline, or plot twist.
Link exchanges.
Link to other sites individually, and propose that they link back to you. Or, you can join banner exchanges, like Rubifruit, or collectives, if they're seeking new members, like Spiderforest.
Interviews / panels / podcasts.
Never turn down an opportunity to talk about yourself. If you're asked, do it.
Reviews.
Plenty of sites and podcasts out there review comics and webcomics. Send them a link.
Press releases.
Let comic news sites know what you're up to, when something important is up. Publishing on paper, or gonna be at a con? Send out a press release, describing who you are, and why anyone should care. (This last tactic is probably best saved until you've already made your initial entrance and people are well-acquainted with you and your comic.)
Not-Free Self-Promotion.
Ad banners.
Go to Project Wonderful, find a comic similar in tone to yours but with higher traffic, and try to siphon some of that traffic off and get those people reading your comic, as well. Try to keep your cost-per-click ratio at either one or two cents.
Con flyers and postcards.
Kinko's (or even better, GotPrint.net) offers very inexpensive printing services for promotional flyers and postcards. Buy some... I pay about $200.00 for 5,000 full-color, double-sided cards... and leave them in computer labs, comic stores, con freebie tables, coffee shops, anywhere you might find an audience.
Minicomics.
A quick, interesting comic, printed in black and white on a photocopier and passed out at conventions (or any other receptive crowd) might get people curious enough to visit your site when they're back home.
Con attendance / con space.
Show up at a comic or anime con with a backpack full of minicomics and promotional postcards. Actually meet people and talk up your comic, especially to other cartoonists. Pass your schwag out like candy, and try to get feedback and opinions. Drop a pile of postcards off at the free table. If you have something you think you can sell, consider investing in table space. Come up with a popular and flattering gimmick... Sharpie a third eye on your forehead and put up a sign reading DRAWINGS OF YOUR SOUL: FIVE DOLLARS or something... and sign your URL to every picture.
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