NOTE RELATING TO GINGERBREAD ROMS: There have been many reports that using Titanium backup to restore from Froyo to Gingerbread has caused numerous problems for Gingerbread users. Therefore, I recommend that you NOT restore any system data, and that you exercise caution in restoring anything else, when going from Froyo to Gingerbread.
I do a lot of ROM testing and I always wipe all partitions going to new ROMs, or even new versions of the same ROM. This guide assumes that you already know how to install a ROM (for my method for installing a ROM using ClockworkMod recovery, see here: Project Elite; for SPRecovery installation instructions, see here: Project Elite).
For those who are interested in getting a clean start when they load the blazing goodness of a Project Elite ROM, here's how I do it:
1. Purchase the license for Titanium Backup (TiBu) if you don't already have it -- it's worth it -- and go to Batch and use Backup all user apps + system data.
2. Once your TiBu backup completes, open ROM Manager and select BackUp Current ROM, then give the backup a descriptive name for your current ROM (this will do a nandroid backup of your current setup, which will be stored on your SD card under clockworkmod>backup). I always try to catch it after the backup completes and the screen goes black (before the M logo comes up), immediately opening the keyboard, pressing and holding the X key, so I go straight to recovery rather than booting up again.
3. In ClockworkMod recovery after your backup, do a factory reset (wipe data and cache), then go into the partitions menu and format boot, then format system, then go to the advanced menu and Wipe Dalvik Cache.
4. Install the ROM zip file following the installation instructions.
5. Once you boot up, touch the top left, then the top right, then the bottom right, then the bottom left (in a square pattern) of the screen to skip Google sign in.
6. Make sure you have Unknown sources checked in System>Applications.
7. Open TiBu and go into Preferences and make sure you have Force to same location checked (this will then make sure your apps and data are restored to the device and SD card, as applicable, exactly as they were when you did your backup).*
8. Still in TiBu, go into Batch and and start with Restore missing apps + all system data -- we'll do apps and data in the next step. When you restore missing apps + all missing data, here's the important thing: press Deselect all and ONLY check the items in GREEN (this will restore all your call log, SMS/MMS, accounts, wifi and bluetooth connections, etc.). If you restore the stuff in red, you're restoring system data from prior ROMs, which could cause you all kinds of issues. NOW, here's another very important caveat: some people have found that restoring [SETTINGS/BLUETOOTH] Settings Storage 2.2.1 from "Restore missing apps + all system data" can cause issues (for example, if you had a password (PIN) to unlock your phone when you did your TiBu backup and you restore this setting on your new ROM, you may find yourself completely locked out after you set your password on your new ROM and you reboot -- which means that you'd have to wipe and start over again; also restoring that item from one version of PE to the next, or coming from LFY to PE, restores all the Elite Options data, including color choices in Interface, etc. For some folks, this has caused FCs in the SpareParts portion of Elite Options). One additional thing: you won't see some of your data (call log, SMS/MMS, etc.) until after a reboot -- I generally wait until Step 11 when I need to reboot to make my backup anyway, but it's certainly fine for you to go ahead and reboot now if you want to ensure that all your data is there.
9. After you've finished restoring the items in green (other than the one in my caveat above unless you're feeling adventurous), run Restore missing apps with data (these will all be missing user (non-system) apps that are in WHITE text) and go through the list to make sure you've got ONLY the items you really want to restore checked. DON'T restore Liquid Tweaks, Liquid Tweaks Helper or Spare Parts if they're on your list. Take your time going through this list, and make sure you have unchecked system apps from prior ROMs or older versions of the same ROM. In addtion to the ones I've already mentioned, some examples from my list that I make sure are unchecked are things like Calendar 2.2, Android Keyboard 2.2 and Music Mod 1.8. These may differ from what's on your list, but hopefully that gives you the idea of the kinds of things you're looking for.
10. Go to Settings>Accounts & sync, Add account and sign in to Google. Make sure Auto-sync is checked and that Sync Contacts, Sync Gmail and Sync Calendar, etc. are checked, and allow them to sync. It's possible that the Accounts restore in Step 8 already restored your account, but you should still look to be sure all your items are checked.
11. Make a nandroid backup of this setup BEFORE you start adding themes and kernels -- that way, it's easy to revert back to stock and start over if a theme or kernel messes you up.
If you run across any apps that restore from Market without your data, because you missed them in your TiBu restore, just go into TiBu and go to filters, type the name of the app, and restore data only.
The only thing left is that you'll likely have to set up your widgets again (at least I do in Sweeter Home 2) and you may have to restore some of your ringtone choices in Contacts, Gmail, Calendar and Gtalk.
*One other note: if you have Swype, I've found that TiBu restore doesn't always work right. If you have the official beta version that you're reinstalling from the Swype beta website, you'll want to reinstall the Swype installer, then Swype FIRST, before your full TiBu restore, to avoid the memory issues that often plague Swype installs. This means that you may have to sign in to Google at this point, or use TiBu to restore just the email app that contains your Swype beta email (for me, that's K-9 Mail), before you go on to the rest of the steps. Don't worry, since you used the square pattern to skip initial Google sign-in, your sign-in now won't trigger an unwanted auto-restore from the Market.