I also recommend AppShopper. Take advantage of the Wish List feature on the App Store too, just because it's easy to manage. It won't notify you or anything, but I usually kick stuff off the list as I get less interested in it.
Lately there's been weekly deals listed in the bottom part of the App Store, stuff like IAPs in apps going on sale (like to unlock the full app, camera apps for example [Like Replay]), and some game deals.
As for patterns - September, December, and January are the big ones. But there's always stuff during the year, and if you hang around places like MacRumors you'll hear about some of the outside promos developers get together and do.