By some project Wonderful ads. They're cheap and easy to use.
If you want to retain that position and get new readers it's the best way.
Yunno, I just tried that a couple of days ago! I was initially underwhelmed by the twenty-something unique clicks my ad was averaging, but I can see where it could be beneficial in a long-term perspective. If some of those uniques become regular viewers, and I kept picking up a couple dozen every day, for an extended period of time, and had several different ads on several different sites... yeah, that could add up to something substantial over the course of a year or two...
...As long as I'm willing to pay for it. And as long as I'm willing to constantly churn out new ads when the old ones get stale. Which I'm not. So it probably isn't worth it for me, but for a comic just trying to break the top 100 I'd definitely recommend going that route. You're right, they're cheap, easy, and I'm sure effective enough given time to work.