Lametastic question: how do I back up?

Mkay, I've been Simming a loooong time and have NEVER backed up. Since the fall, I've been having catastrophic neighbourhood losses and lose all my Sims etc. every time it happens (I'm *thinking* it's because I have less than 1 gig free. Moved a bunch of crap so I now have 5 g, gonna see if that makes a difference. Gawd, if it doesn't...) and I'm getting TIRED of starting all over again every two days. So. Maybe I should start backing those babies up. How do I do that??

You're not alone in you catastrophic losses. I feel like there's been a sudden surge in those lately. You're smart to back your sims up-it's our only defense!
There's a couple of ways you could go about this. The first way is to back up to a CD or DVD (probably going to have to be the latter). Here's what you do:
1) Insert a blank DVD into your drive
2) What you're going to want to do is find your Sims 2 folder. To back up your actual sims, you're going to want to back up your neighborhood folder-each folder inside represents one hood. N001 is Plesentview, N002 is Strangetown, N003 is Varonaville, N00(any other number) would be your custom hoods, and G001 is Riverblossom (if you have Seasons). To back up the hood, simply drag it's folder to the empty DVD.
3) You might also want to do the same with your downloads folder, and your saved sims folder if you make any custom content in bodyshop.

If you have an external hard drive, you can do the same thing with the hard drive in place of the DVD. If you are running Leopard, just set up time machine and let it work for you. If not, just drag the folders to the hard drive just as you would the DVD.

Some words of advice: Make sure you remember to back up frequently! Also, if you're using DVDs, be sure to clearly label them with the date you made the backup.

Just to be sure everything is kept intact, I backup my entire user/documents/EA Games/Sims 2 folder. That way I know all my files relate to each other, etc.

Copy to a dvd or another hard drive, like purplestarz said.

Or get Leopard and an external drive (Time Capsule?) and let it all be done automagically for you by Time Machine. Horrible thing happened in your game but your mouse clicked on save before you could think better of it? Just go back to the Time Machine backup 2 hours ago, before you started playing. I simply cannot say enough good things about Time Machine. You need a backup drive about 50% larger than the drive you are backing up.

I do about a weekly backup onto an external hard drive, but every day I just duplicate my neighbourhood file (the neighbourhood that I am playing with) and move the duplicate into another file in documents (not in the ea games file). This was enough of a backup the last time my neighbourhood disappeared.

Yep, that'll work, too. The point is that you do need to backup your neighborhoods regularly, because they can and do explode. Sometimes for no reason at all. When that happens, if you don't have a backup you're left with a smoldering heap of unuseable data.

I did that and now I can't put my neighborhood back on the game. The only option I have is the "create neighborhood"

What did I do wrong??

Sounds like the folders got moved, not copied. Have you tried to copy BACK into the orginal folder your neighborhood files?

What's IS in your Sims 2 folder?

I tried to move, copy + replace, delete default folder + paste backup backup folder, copy + paste each subfolder and none works. Now I got all the default hoods back, but I can't put my sims or houses back in the game.

my files are in the default location (documents/ea games/the sims 2)