Well, if you really want people to pay you for ad placements, I recommend adding some value to them first - retweet them, add insightful comments to their blogs, share their content, promote their stuff. This also gives you an opportunity to gauge how your audience reacts to it, which gives you clout when you start asking them for money.

But, I'd also caution you around your assumption that paid ad placements will make your business more money. Yes, you get revenue from the advertisers, but if this is a turnoff for your audience, you lose subscribers, no one clicks and advertisers drop off.

The money is in the list - so make sure this is something they want.