When and why should I revert to my service provider's base ROM?
Die-hard flash fans will revert back to the SP ROM on every upgrade, to avoid Extended ROM corruption. This isn't absolutely required, but most die-hards will recommend it. I recommend it.
I have a G4...HardSPL, SoftSPL, what's the difference, and why should I care?
Because of previous question, I usually recommend using SoftSPL for most people, but the RUU doesn't always work with everyone's ROMs. SoftSPL (one-shot) HardSPL (semi-permanent) Removing HardSPL
The cook's installer didn't work on my phone, where's a good one? Mun's RUU(works for almost everyone...just drop the nk.nbf into Mun's RUU, and flash away...)
What are the must-have computing items for flashing a phone?
The most reliable method is with a Windows XP machine loaded with Activesync 4.5, and connected via USB cable. Don't try it on Vista or older Windows versions, as it usually fails, and might even brick your phone. If you're a Thunderbird/FinchSync user, don't even think about it.
For your mobile, get DotFred's PPC PIM Backup tool, and load it to your SD card, so you can back up your contact list, call history, etc. for easy personalization. Note: Remove your SD card while flashing; it has been known to corrupt flash process, and occasionally to wipe out your SD card. If that's where you're backing up your PIM data, you'll be pretty bummed.
What SD cards can I use?
Anything that is clearly non-HC (high capacity). Generally, if you stick to 2GB or less, you won't have a problem with this, but if you go looking for 4GB or higher, avoid anything with the HC label (i.e. SDHC); your hardware cannot read it, and it may corrupt data or even possibly (though rarely) brick your phone.
Which ROM is best?
Wrong question. Right question is: which is best for YOU? Try this thread for helpful pointers.
If you read all of this and you still have questions, feel free to reply.