So I tend to reformat very frequently (not because I want to, but because of hardware changes), but I've got this sort of system down for how to make things easy. Some of you may already do things like this, or you have your own thing going, but thought I'd share these tips, for those who would like some input.
Keep your files on a non-Windows hard drive.
This may sound odd, but after reformatting the first 2 or 3 times, I found it to be a complete pain in the @$$ to back up everything that was on my Windows-installed hard drive, then transfer it back after reformatting. So what I eventually did was take all my media, all my documents, game saves, program install files, etc., and put them onto a non-Windows-installed drive. I have a 500GB drive that has a My Documents directory set up exactly how Windows sets it up. So under My Documents, I have My Pictures, My Music, My Videos, Saved Games, etc. This comes in handy, because you can actually configure the My Documents link in the Start Menu to use a different directory other than the one on your C: drive. Just go to your start menu, right click on My Documents, go to Properties, and you should see a directory path that you can change. Same for the My Pictures, My Music, etc. This eliminates hours of transferring lots of files.
Additionally, this means that your primary hard drive with Windows installed doesn't need to be enormous. You can use a smaller drive, such as a 160GB drive, and use your larger drives for storage purposes. On a Windows drive, I've never exceeded 150GB of installed programs, including the dozens of games I have, an entire Adobe Creative Suite, and more. If you use a 500GB drive when you're not going to use more than 100GB, that's a lot of space wasted. Another option here is to partition the Windows drive, and use the non-Windows partition to store files.
Back up your drivers onto a disk or hard drive.
Kind of self explanatory, but I found out the hard way. Download the motherboard drivers from your manufacturer's website, the video drivers from ATI or nVidia, your sound drivers, program install files, etc. Even if they're outdated, it's easy to install those and update them later. I had an ASUS motherboard, lost the driver CD, and it took me forever to realize that ASUS puts the network drivers on the chipset driver package, instead of having a separate network driver download.
Also back up downloads of Service Packs. These are large files (100s of MBs), and it's not necessary to redownload them, if you can just back them up instead.
Common files that I recommend having a backup of:
Service Packs (just the most recent one)
WinRAR/WinZip
Spybot/AdAware/other anti-spyware programs
Drivers: chipset, video, audio, network, DirectX (updates monthly, so make sure you have the most recent one)
Flash and Java
Firefox (or other browser you use)
Back up your downloaded games (the legally downloaded ones)
Sometimes, for programs like Steam or EA Download Manager, you may be able to copy the downloaded game files, and transfer them back to your C: drive after reformatting. This would save you days (maybe weeks) of having to redownload the 25 games you have on Steam or the 10 you have on EADM. I'm not sure that this will work all the time, because I've never actually tried it myself, but I've read that other people have been able to do this at least with Steam.
So those are just some tips that I've found to be very useful over the past couple years. It helped me be a more efficient Windows user and I think some of you may find it useful.