The best ways to monetise your apps include the following -
Make your app a Paid version If your app is really popular, the best way would be to make your app a paid one. It has the advantage take of upfront cash per download.
Mobile partnership You can think of securing partnership with well known brands. You can also think of simply getting your app sponsored by a major advertising firm. Please note it would be possible only if your app is of high quality and gets a good following.
Sponsored contents Create some content sponsored by brands, like virtual gifts. A better example is Temple Run 2 which used Usain Bolt as a limited time character.
In-app advertising Promoting an ad within your app is one of the best methods to get better monetisation. Sign up for an ad network, download their SDK , select the kind of ads you would like to employ and deploy the ad.
Interstitial ads Interstitial ads are the ones that fill up your entire screen at specific strategic intervals. You can set it up to show ads when exiting the app, or at certain intervals.
Banner ads These generate lesser revenue. Though they may be slightly looked down upon as spammy, they can generate good income.
Use app walls App walls show multiple apps and could be good channel to generate a worthwhile revenue. They may not generate on the similar lines as Interstitials.
Video ads Video ads attract highest CPMs. These are the biggest generators of revenue if employed properly.
Live....and Let Live!