After my post on How To Really Quit, I scanned through the Emacs section of Brian Zwahr's Dev and Such blog. He's got a ton of good Emacs tips there (many from his previous Blog Emacs Journey, which he's merged with Dev and Such). You really should head on over and take a look if you're not already familiar Dev and Such.
I don't want to reblog everything he's already said but here's a tip so useful I am going to reblog it to make sure it gets as wide an exposure as possible. I often have a hard time remembering all the, say, register commands. I know they all start with 【Ctrl+x r】 but I find it hard to remember the suffix of the various commands or sometimes even what commands are available. Zwahr's tip is to type the prefix and then 【Ctrl+h】 to get a list of the commands.
Oddly, the example he gives is exactly the one I always have the most difficulty with: the register commands. Suppose I want to save the point to a register but I can't remember the exact command. I just type 【Ctrl+x r】 and then 【Ctrl+h】 to get a list of all the commands that start with 【Ctrl+x r】. Since I learned that trick I've used it several times. Very very nice!