I'm not going to list the different options you have in backing up your MS because a)you probably already know, and b)other people have said it so much better than me.
Check out the links:
http://lydiakang.blogspot.com/2010/03/backing-up-your-writing-is-it-secret-is.html
http://querytracker.blogspot.com/2010/03/backing-up-know-your-options.html
Up till now my basic knowledge on saving my work has been a USB stick, my hard drive and a printed copy of my MS.
I have two USB sticks and I save on both. Why? Well I have this panic that I may loose one, or it could get corrupted or the million other things that can go wrong. Plus my parents bought me an 8GB one last Christmas which puts my 2GB one to shame.
The printed copy scares me. Do I really want to go through the trauma of developing carpal tunnel syndrome by re-typing it or scanning it back into the computer, no.
Did I even know you can send your MS to yourself via email? Nope, but I do now and I have done. I'm loving the extra space on Gmail right about now, thanks Lydia for that info.
I'm probably preaching to the converted but I'm going to repeat the mantra. Back up, back up, back up. The disease spreads quickly folks, prevention is always better than cure.Now I'm off to back up.