First thing to improve your approval rate would be to make sure your phone browser is accepting all cookies. This can be important, although some apps will approve without cookies some MAY still require it. So if you have ever wondered why a certain app won't approve it could be as simple as the advertiser not receiving the cookie data so therefore no approval. Simply choose to accept cookies and more apps should approve.
Sound familiar? It should, people are always saying to clear cookies when completing P2S offers but the same also applies to mobile devices. Its simple really, even on a mobile device data is still stored by advertisers. Some advertisers may only pay for ONE app installation even though they have multiple offers, this is because to them you are already a customer and have already been paid once. Clearing your cookies/cache will make you appear as a NEW customer so you are more likely to get paid.
Install and Use
Some apps will pay for simple installation (yay!) but others are sneaky and have requirements (boo!) some are easy like "sign up" whereas others can be time consuming "reach level 10". Its really important that you USE THE APP! Even though they may not say you need to use it it can definitely help.
Been paid? Great! You can now repeat the clear cookies/cache step and complete more offers! You can also un-install the app if you like. If it hasn't yet paid then keep the app for a few days, use it if you like. If all else fails then count it as a loss and move on, not EVERY app will approve, but if you follow the right steps then most apps should approve.