Such as Google Adwords or Overture or Microsoft Ad Center?

I dabble with them a bit so I'm no expert but would love to learn any secrets anybody has for targetting keywords and getting good quality ads to reduce your CPC pricing.

I have used them in the past with my product based business, but nothing with forums.

I've got one strip of google adsense adverts on my forums pages (all pages) The integration worries me as I'm getting a very low CTR. I'm hoping as I put a new skin on the place and the visits increase so will the CTR!

Nope, I haven't tried.