One is to allow ads. Google AdSense is probably the easiest. If you're going to get serious, consider using AdSense Channels, so you can see what people are clicking on. Knowing what key words are producing clicks is vital to making decent money. Yahoo also has AdSense, I think. AdSense pays cents-per-click. The alternative to CPC is a system I'm not sure about called CPM. I avoid even discussing it because the people who do it include Casale, Adify, and other people I have always considered malware company.
One is to be an affiliate, meaning to have a link to a retailer, such as Amazon, so that you get a tiny fraction of every purchase made by someone who accesses Amazon through that link. Other than Amazon, look at Commission Junction, FlexOffers, LinkShare, and ShareASale.
I don't know the difference between affiliate programs and referral programs. Swagbucks and Sharebuilder are referral programs I've heard good things about. Again, though, I don't get how they work differently from affiliate programs.
You need publicity. I know people who swear by Gather. Others talk about blogtours and blog carnivals. You want a lot of links back to your blog, so get in all the directories you can. Also, put out lots of Squidoo pages that have links back to your blog. If you have the nerve for it, put a link in your signature and comment a lot on other blogs.
You need Google Analytics or something like it to see what pulls people to your site. Then you need to give people more of that. Find a "keyword tool" site and find out how many people are looking with specific keywords.
I keep hearing about Search Engine Optimization. As best I can tell, this is a glorified version of picking key words by their popularity.