use interstitials as much as you can, for me at least, they make way more money than banners (more than 5 times). For 5819 interstitial requests yesterday, I made £28.85, RPM of £12.51. There were 2306 impressions. Be sure to check that your ads are actually being shown, not just requested, the number of impressions should be high, not just the number of requests.
Just to add, I don't pester the users with lots of ads, they'll typically see two or three in a play session.