I've done these in the past for various e-commerce projects. Most of my colleagues who sell online do the same:
Don't underestimate the potential of a well-optimized site and a good SEO campaign. Focusing on content and building links can simultaneously create buzz and bring more and more visitors to your site. Check out SEOMoz if you're new to this.
Get the word out there in the form of blog posts, guest posts, articles, discussions, etc. This builds a gradual but consistent flow of traffic to your site and also helps with SEO. Use Google Keyword Tool to figure out what people are searching for, and write about it. Make sure your articles are interesting enough to read, though.
Pay-per-Click ads through Google and other networks can make you a lot of money if you have a conversion-optimized website and you keep your costs lower than your revenues. The key here is to constantly measure and optimize so that you're making, not losing, money. What's great about PPC is that if it's profitable, it's easy to scale up. Beward, though, as it's not appropriate for every business.
When you sign up for these networks, you agree to pay a commission to anyone who gets you a sale. The network will let its advertisers know about your offer, and people will sign up to join your "Affiliate Program" (you can screen them individually, usually, if you like). For example, say you agree pay 50% of your profits for any sales brought in by your affiliates. They may run ads, create landing pages, and ultimately guide users to your website for you in exchange for the commission. This can be profitable, but also a shady business, so read up on it before you dive in.