We all know that all links are not created equal. With that in mind, how should you spend your link building time?
I've compiled a general list of common ways to get links, and I've categorized them into the most effective and powerful down to the least effective and powerful, and about how long it generally takes to get each type of link.
1. The Gold Standard: Dofollow, anchor text specific links in the body of an article. These links can be from a number of different sources, and can be found in a number of different ways. There are two common ways to get these types of links. The first is to submit an article to an industry magazine and hope they publish it with your links intact. Another popular way is to contact bloggers in your niche and ask them if they would publish a unique article (that you have written specifically for their site, and includes anchor text links to your site). A lot of bloggers appreciate well written, unique content, because content is like money to bloggers. Finding new sources of good sources for this type of link is painstaking, and getting one good link could take you anywhere from one hour to three hours---but it's worth it.
2. The Silver Standard: Regular anchor text links in a blogroll or resource page. These links are easier to find. You can find bloggers who do three-way link trades pretty easily. One place to find such trade opportunities is the Digital Point Link Trade forum. Also, by doing a Google search for '[keyword] resource page' you can find plenty of sites that have lists of resources related to your niche. The hard part is contacting these sites in way that will get a positive response, and most of the time they will require a link back to their site. I normally don't suggest doing reciprocal link trades unless your sites are 100% relevant to each other, and it makes sense to do it. You could use one of your other sites to send a link to a trading partner, who then will link to the site that you want to get promoted. Expect to spend at least a half hour per link.
3. The Bronze Standard: Article Marketing. Article marketing has the potential to get you good rankings, in the event that one of your articles gets syndicated on a popular site. This could send targeted traffic and give you some SEO benefit. Most of the time though, your articles get duplicated all over the place, and it's a shot in the dark if your article bio box with your links will stay intact. As with all links, the more diverse your portfolio of links is and the more natural your backlinks look, the better chance of staying out of any search engine spam filters. You can easily get five links in an hour doing this.
4. Blog comments: The reason I even put this type of link on this list is because I've seen direct benefit from commenting on popular blog posts. Not only do you get direct traffic, but it helps with your rankings. And Google's smart enough to know that blog commenting is never going to go away, and is a useful feature, and therefore, a blog comment is a useful link to your site. Keep in mind that I do not endorse 'spam commenting'. If you can't add something valuable to the blog post, or offer a different point of view, don't comment. Period. You can get at least ten good comment links in an hour.
5. Social Bookmarks: You can get dofollow anchor text links to your site by just bookmarking them at sites like Digg, and other dofollow sites. The reason this is low on the list is that everyone and their dog is doing it, and these links don't have as much weight because the main page of these sites (besides the big ones) don't have much Trust Rank or Page Rank or whatever your pet phrase may be. But again, search engines expect you to have a diverse number of links, and social bookmarking is another popular type of link these days. Twenty an hour is a reasonable amount.
6. Directory Links: These will never go away as long as the internet is alive, so you might as well continue to use them. Some words of advice though, if you are going to submit to a directory, make sure that more than just the home page of the directory is indexed, and that the directory has been around for at least one year. I'm sure there's more advice from people who use directories a lot, but those are the basics. If you're good and you've got your form fill plugin for firefox, or Robo form, you could pump out 30 directory links in an hour.