Apple's unloading iOS4 (that's the new name for iPhone OS, in case you didn't know) tomorrow, which means you should get started tonight.
Download iTunes 9.2 and run a manual backup (right click on your phone in iTunes, hit backup). This way you won't have to spend time doing all these mundane housekeeping tasks tomorrow. And if you're looking for what's new, here's a good place to start. Then you can head here, which adds more minor features we found while actually playing with it. [iPhone]