There are many ways, just make sure you dont over-do it.
1. Google Adsense
2. Amazon affiliate, get the widget and just copy and paste the html into your blog
3. Ebay affiliate
4. clickbank, join and get hoplink from the product you want to promote, either use hyperlink as intext advertise (remember to add no-follow tag), or put in side bar;
alternatively write review for the product.
5. join textlinkads or textlinkbrokers to sell text links, the risk is that they use do-follow tag, and your blog PR will go down.