Step 1: Requirements
- A Raspberry Pi
- A Windows PC with Notepad++ installed http://notepad-plus-plus.org/download/v6.1.8.html
- A small heatsink and thermal paste (If you're going to go past 750 or 800mhz)
The heatsink can be bought from Jaycar (if you're in Australia) or Radioshack (If you're in America) or where ever else you can find a Pin grid array (PGA) heatsink. http://www.jaycar.com.au/productView.asp?ID=HH8580
Step 2: Create a Config.txt File
If you're feeling bold you can adjust them a little higher, but if you're going to go to 900 you definitely need a heatsink as well as bring the GPU down about 50mhz to make up for the power being supplied to the unit.
Once this is done, save your file to the boot partition of your SD card as "config.txt". The boot partition will be the only partition that shows up on Windows usually(about 60mb in size) - if it doesn't show up, you'll have to put your config.txt on a usb and transfer it onto the SD card with a Linux machine like Ubuntu.
Step 3: Finishing Up
If it doesn't boot, or has a kernel panic, don't worry - just edit the config.txt file and bring all the values down a bit until it does boot. Remember the default CPU clock frequency is 700mhz.
You should notice about a 15-20% increase if you run any benchmarks, more if you bought the heatsink and took it to 850mhz or 900. If you take it over 900 without a boost in voltage, it probably won't boot, and conservative old me isn't going to ruin my warranty by screwing with such things. If you run into any problems, then you can just wipe the config.txt file to get it back to normal.
Sorry for the lack of pics on the heatsink, I ran at 800 without one for a little bit but as soon as I get one (probably this weekend), I'll upload it.
You can do all this on the pi itself too, and just reboot it.