If you are new to rooting, you might want to be aware of some general guidelines for various root-related activities that might help you avoid some trouble along the way.
- Have a fall-back / recovery plan before taking any "scary" (risky) action. In other words, know what steps you'll need to follow if you encounter a problem.
- Make a backup before you alter or remove any system-related apps or settings. If you can't make a full, restorable (i.e., a Nandroid) backup, then don't make any changes--doing so without a backup is asking for bootloops or soft-brickings.
- If your Android device does not support a custom recovery or the ability to make a full device backup (i.e., a Nandroid backup), then you should avoid making any system-related changes that could affect the ability of your device to boot (i.e., modifying or removing system apps, etc.).
- Verify the integrity of any files you might flash. This means checking the file size and (MD5) checksum on the platform from which the file is being flashed. If you are flashing from your device, you should check its integrity there there's no guarantee that it matches it original source unless you verify that. If you are are flashing from a PC, you should also check the file's integrity from there, too.
- Be sure to carefully follow the ROM (or theme) dev's installation instructions. If wipes are required, be sure to not miss those steps. The sequences of steps is often very precise and following them correctly will help ensure a successful installation.
- When/if you overclock, be sure to not enable the "set on boot" option until you are sure your device is stable at the selected speed. Also, don't forget to to set a failsafe temperature profile/setting to keep your device from over-heating.
- Also, take care when first using a tool like ROM Manager. It makes doing some root operations very simple, but you might want to become with conversant with how to operate and navigate directly in ClockworkMod custom recovery. This way, you will be prepared for the possible eventuality when you really need to manually operate in ClockworkMod custom recovery. So, your first forays with flashing ROMs might be best done manually.
Let me know if you have any other things to add or expand on in the above.