Track your sales
[Stub article -- this needs lots of work.]
There are a number of tools available to help you track sales. I haven't directly used any of them, so can't make a recommendation, but here is one developer's review of two popular systems:
I also have an open-source app for the phone that I use called AppSales. I build it for my clients who ask.
Of course, you can do whatever data-munging you want on your sales-trend reports, which can be downloaded from your iTunesConnect account, depending on how resourceful you're feeling.