How do I insert an ad after the first post in a thread?

[huss]

Let's say I want to insert an ad or somesuch in after the first post of every thread. How would you go about doing that?

You would stick it at the bottom of the while loop in showthread that displays the posts.

I've taken a look through the code, and frankly it's beyond me how to do this. I think this is very useful for ad driven sites and was wondering if you could give me more guidance? Or should I ask in hacks?