- Ad position - There really only two ad placements that matter, the top banner ad and the side one. I almost never got to a comic featured on a side banner, though I'm not sure why. I think a lot of it is that very few comics are actually designed for the vertical (Kawaii Not being the rare exception) and so the side banner never feels right.
- Flash - I have it turned off on my browser normally, so if you have a flash ad, I'm not seeing it. I imagine a lot of other people do the same (and a lot more block ads entirely). If you're going to have an animation, stick with gifs.
- Replicated Strips - A lot of ads have full strips from the comic in them (usually Daily Comics). I skip those generally. Ads should entice a reader to read the comic, not to start them reading before they get to the site. I suppose if the joke is VERY funny, it might still draw someone in, but if it's only passingly humorous, you could have problems. I would suggest fitting the art to the ad, making something specifically for the ad itself that will get across the point.
- Wordiness - AKA: Too much text. I don't want to read it all. There was an ad I remember that had a LOT of text on an animated cycle and it took a bit to watch the whole thing. I decided not to bother reading the comic. Keep the text to a minimum, the title is often more than enough.
- Unrelated to the Comic - I know controlling which comics an ad gets put up on is somewhat difficult, if not impossible, but if the ad is for a comic that has NOTHING in common with the comic the ad is on, I'm certainly not interested in visiting the comic. If you can control this, try to advertise on comics with similar themes (Sci-Fi, roommates R us, fantasy) or similar artwork style (stick man, manga, etc) in the beginning as it'll build a nice base of readers (says the guy who doesn't advertise his own review blog at all).
- The "Catch Phrase" - I really can't think of a better term for this. It's the short phrase that tries to draw people to the comic, and they generally don't work very well in my eyes. They're supposed to tell you about the comic, but typically they're what the artist thinks the comic is about rather than what it is about. Keeping it brief helps, and making it as actually descriptive as possible will as well. Yeah, that sounds contradictory, and it probably is, but I generally don't like them so that might be my way of discouraging you.
Well, enough of that kiddies, see you next time.