In general, I figure that you should start with adsense (because it's the simplest to add in and get a lot of relevant ads). Then you want to look for more focused networks, as the general rule is you'll get better performance the more focused you are.
I did a short write up of using the major players over at http://www.adbalance.com/ad-networks/
Depending on your niche and traffic though you may be better either directly selling to advertisers (serving/tracking ads through OpenX, Google DFP, or OIO Publisher is pretty straight forward).
Again depending on your niche you might find a vertical ad network - there are these for food, health, religion, music, sport and pretty much any other popular vertical you can think of. They tend to get better ads at better rates - but obviously have higher entry requirements.
I'm not sure the amount of traffic that you have, but OpenX has served me greatly on a similar project.
It's important to note the market your app attracts. If it's highly sought after and lucrative, you may be able to find a small advertising network focused on your niche. Something to think about.