First, and most important - there are better ways to make money than affiliate programs. Start by contacting ad agencies like Flycast, Burst and Image Networks. These companies sell CPM advertising - you are paid whenever banner is displayed, regardless of whether uses clicked on it or not. This is the best way to make money with the site - all you need to do is build up your traffic and profit will grow too.

The alternative to these companies is 'pay-per-click' agencies, such as Valueclick, but my results with them were so negligible, I am not even going to talk about them.

Anyway, once you've signed up with the agency and have banners running, you can look into secondary source of income - namely affiliate programs. Here's several resources where you can find excellent programs: Commission Junction, Reporting.net and ClickXchange.
They have good selection of merchants and friendly user interface. There's also ClickTrade and LinkShare, but their websites are often slow. Also, LinkShare has the most annoying popups.

As you explore these services, you will notice that there are several kinds of affiliate programs. The best kind would be CPC - you get paid whenever user clicks on a link. Not as good is CPA (cost per action) - this is where user needs to register for a service or download a program before you get paid. And finally, the worst kind is 'pay per sale' - this is where you get a flat fee or percentage of sale.

Once you've selected the program, you will have a chance to select banner or text link. In my experience, many banners don't work well - the fact thay they look pretty and are colorful may activate user's "mental ad blocker" and he will skip them when browsing your site. However, some banners seem to work ok, especially if they are related to your site subject.

The text links seem to work better - they don't stand out from site contents as well, so user is more likely to click. Note however, that you don't want to use misleading text links - it's an easy way to lose reader loyalty. If you don't care about retaining the users, try ClickHereToFind - they allow and encourage use of 'deceiving' links.

As you are building the links, you may notice that most of them have disclaimers like "Do not modify the code or you will die slow and painful death". I believe that disclaimers like this are mainly meant to discourage beginners who might mess up the affiliate code and then complain that they are not getting referral money.

The truth is, customizing the original code is where the money is. For example, take a search engine. They may allow you to put a search box on your page - but why would your reader want to use it ? It's another matter if you prefill the search form to look up things related to your site subject matter - this will immediately boost your click-through ratio, as well as improve your site.

That about wraps it up. Use these tips and you will be well on the way to building a solid secondary income!

Related links:
• Submit Me - free submission to 15 major search engines
• You Surf - They Pay - review of 'get paid to surf/read email/listen to the radio' programs