Aside from buying from the Apple App store, I always make it a point to browse MacUpdate. They have pretty good sale items there as well as bundled apps. The last one I bought was the spring bundle which had Devon Think (a productivity app mostly used for writing and academics), Parallels Desktop (a virtualization machine for Windows), DiskAid (an app that synchronizes iOS devices with a Mac without going through iTunes), and other 7 other goodies for the great, great price of $50. It was a particularly good buy for me because Devon Think actually costs $50 on its own so the bundle had really made it a good decision. So if you're waiting for a particular app to go on sale, just wait a little bit more because they'll probably put it in a bundle in the future.