If a site receives a lot of traffic, what would be the best way to cover future traffic costs or hosting fees with (PPC) ad placement?
I tried with Ad-sense last few months, but received not one click all the time and I also think that Ad-sense has very strict rules e.g. you may not force your visitors to click. The count of ads is limited, you may not put your ads near to any other links etc.

So I'm searching an ad placement provider where it is possible to generate some income. A system like this: Visitors have to click on an advertisement before they can view or download a file on a site.
That way I would make sure, that traffic will be covered by the ads revenue.
Has someone experience which advertisement company would fit best to my needs? Thanks for every input!