When I started using Admob, my fill rate was around 70% and my erpm was very low. One way I increased my fill rate and erpm was to enable “Age Appropriate Ads” on my apps. I’m not a big fan of the ads, they tend to be flirty PG-13 stuff. Admob defines them as:
An ad is designated age appropriate when the content or service promoted is not suitable for all audiences. Examples of age appropriate advertising are gambling and personals ads. AdMob does not accept ads that promote adult products or services.
Turning on the age appropriate ads doubled my erpm (from $.02 to $.05) and bumped my fill rate to 99.7%
To adjust your filters you need to login to admob, click the marketplace link, hover over your application, click the manage settings button, then click the “Category / Type Settings” link. The age appropriate ads are at the bottom. By default they are turned off.