It depends on what you write about, as you want to ensure your readers on each topic are interested in the relevant links you have on your Hub. Adsense is handy as it will find many relevant links for you. Since I write about Dating I'm affiliated with Friendfinder, and make extra earnings from that. If you're interested, see my Hubpages Profile for the link to the Friendfinder Affiliate sign up page. It's free to join.