There are many ways to successfully incorporate ads. When developing your ad strategy consider a typical user session with your app to guide your placements. Here are a few best practices to keep in mind:
Focus ads where users spend the most time
This makes the most of your monetization efforts.
Set up ad triggers that will reach your entire user base
This maximizes reach and ensures a consistent experience across your user base.
Place ads at natural pauses
This is a better user experience and generally results in higher eCPMs and more ad revenue.
Generally it's best to maximize the unique users that see your ads, and limit the number of ads per user to 5 or so since users are generally most receptive to the first few ads.
Learn more specifics on successful interstitial and rewarded video implementations respectively.