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What do you do when you've sold all your ad space?

Come vote in my poll on v7n's forums: What do you do when you sell all your link space?

I have a number of sites with no ad space left and advertisers asking for ad space. I tend to build another site in the same niche to accommodate overflow, but I think this is doing it the hard way. It would be easier to concentrate on the one site and just jack up prices. I have something against raising prices so I never do it...