Reach out to other apps. There are tons of apps out there that have advertisements featuring other apps. The most effective way of doing this is finding out the other apps target market. This can give you a good insight as to if your app will do well if it was advertised on a certain app. If your app is for business owners then putting your advertisement on an app that mostly 7-10 year olds use, chances are you wont get too many downloads because of that ad.