Now that I've provided you, the reader, with two great blogs that go over link building, I feel incline to throw in my own two sense.
There are a few problems with the posts that I have below this one. The greatest problem is the emphasis on reciprocal link building, and while it does wonders in Yahoo! and MSN, it has been majorly devalued by Google. For example:
On the site that I optimize, those keywords in google whose links were mainly reciprocal, have fallen from the first page, to the third page. THose pages whose links are not reciprocal have stayed atop the SERPS. And as a rule of thumb, anything Google does will shortly thereafter be followed in suit by Yahoo!.
What remains the challenege in link building is finding ways of soliciting large amounts of nonreciprocal links in a relatively short period of time. The best ways to do this is by syndicating content and by offering incentives to web owners other than a link in exchange.
Write press releases, offer to write articles, give discounts or coupons to linkers. What I have done recently (and by me I mean the IT guy here) is syndicated a tip of the day script that anyone can post on their blog or website that just so happens to have a link back to me on it. This way, the linker gets content, and I get a link. Think of a widget that is perfect for you.
Firthermore, for niches not counting web design and real estate and ridiculous things of that nature, blogs are a major untapped resource for links. Does anything that you sell have a hobby associated with it? If yes, then there are sure to be countless blog communities out there, just waiting to link to you.
I do reccomend old sites as mentioned in one of the posts below here. Old sites are usually owned and operate by some old coot that created it as a hobby, and never realized that he/she could make money off of it. These people are more than willing to link to your site.