Wayne;

Couple of things I'd suggest; if you're considering advertising on a particular site, take a look at who else is advertising there. If you can, contact the advertiser and ask them if they're happy with their results.
Beyond that, it's test, test, test. Run the same ad on two different sites for a week. After a week, cancel the deal with the one that's not performing as well, then take that money to a new site. A week later, same thing. Keep going until you get the results you want.

Now - depending on your site, your ads, your budget, a week may not be enough. But you should constantly be testing, and challenging your results.

The other thing, with regard to traffic: Ignore things like Alexa rankings. Alexa's rankings are pretty much universally dismissed by those "in the know", because their rankings are based solely on the surfing habits of users who have installed the Alexa toolbar in their browser. Being #22 on Alexa is like being #22 with people who surf while eating watermelon.

As always, that's off the top of my head. Hope it helps.

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