The Most Hated Web Experience, and How to Do It Right (If You Have To) is a good article on pop-ups. Thankfully they’re almost dead (although TripAdvisor seems to use them extensively & personally it puts me off the site!), since people hated them, they didn’t work in engaging people and people are now using pop-up blockers.
The article discusses modal overlays, which are new-school type pop-ups that can be set to appear after a specified time or when a user performs a certain action…marketers have found that these work. So if you’re going to employ modals, read the article’s suggestions on how to implement them properly and effectively.