When it comes to dealing with marketing you have several options to consider, and you need to debate their use when it comes to dealing with your comic. Keep in mind that not all solutions will work for every person; it requires a certain mindset and a lot of time to take advantage of all of them.
Forum Postings: Discussion forums are places where people meet online in order to discuss pretty much anything. Posting in a discussion forum is great; not only does it allow you to ask questions, it allows you to demonstrate that you are an expert in something, thus earning respect, but it also allows you to advertise you advertise your comic in the signature. Better yet, as long as you make sure that your posts are relevant, it's not spamming! Better yet, they can be archived by search engines, so that you can create more backlinks for your projects (and if the forum is relevant to your comic, or is one for comics, it helps your search engine ranking!). Keep in mind that it works best if you post a lot, so limit forums to only those that you know you can post with some decent frequency.
Blogs: Better known as ways you can post whatever you want to, they can be tailored to create relevant backlinks to your comic, and demonstrate your incredible knowledge of something. Keep in mind that you need to post with some kind of frequency in order to make it work, and you need to know about what you are writing of, or people won't go to your site, thus voiding its use to you.
Advertising: Keep in mind that you need to advertise occasionally. Keep in mind that you may have to actually pay for it, but it can be a great way in get people to come to your site. Project Wonderful and Speechbubble are your friends here. Really debate advertising on Penny Arcade; they have a special term for what they do to servers (“wanged”) as they send a million visitors to your site and scare search engines.
Word of Mouth: Don't forget to advertise your site in the real world. If you're at a comic book dealer, let it drop that you have a webcomic, especially if you have collected it into a printed form. Not only can you possibly sell more of the printed version, but you may get a link out of it. Also, you can place posters as well as advertise on bulletin boards. If you can design a decent press release, you may even get local news to sit you down for an interview, which not only adds to your ranking as an expert, but can make you a local hero. In short, there is a practically unlimited potential out there; use it!
Merch: Besides the obvious profit potential, finding ways to create merch(andise, if you really needed it spelled out) also spreads the word, especially if you provide a link back to your comic. I recommend working with “print on demand”; these programs only create the merch when it is ordered, so that you don't need to guess at how much you will need and they will also ship direct to your customer, simplifying your life. Some places to look at are Zazzle, Spreadshirt, and Cafe Press, for general usage and 4imprint and CustomInk if you are getting ready for some sort of special promotion or conventions.
Fan Art: Draw a picture of your favorite characters from a webcomic, and send that art (with a link and a thumbnail) to one of your favorite sites. If they have a fan art page, and they list your art, then you get advertising for your webcomic. You can also do link exchanges as well...
Viral Campaigns: The hardest campaign to pull off. You need to be subtle, and create a buzz without it getting out that you are the one creating the buzz. Obviously you will need to hire someone, or a group of someones, in order to pull this off, as well as create some advertising in other areas to drag people in. Not advised for newbs.