I actually started doing it by accident. Someone emailed me and expressed interesting in buying a text link on my site. I wasn't sure how much was a fair price, so I asked what is was worth to him. He came back with an offer that I thought was more than fair, so I agreed.
After that, advertisers just started coming for me out of nowhere. I developed a pricing plan based on demand and space available, and I keep track of them all in an Excel document.
I've had somewhere around 10-15 different advertisers, with no effort on my part. Many of them renew their ads every year. Maybe I was mentioned on some kind of advertising network or something?
Anyway, if you want an estimate of what a text ad on your site may be worth, check this out:
http://www.direct-link-ads.com/
Keep in mind, it's only an estimate. I actually sell mine for higher than the site thinks it's worth. The site recommends that Lisa charges about $50 a month for text ads, but Lisa sells them
directly on her blog for only $15. That's a steal! How nice of you, Lisa.