A good online backup solution is something that you setup once, and forget till the day you need it.
Personally I use a service called CrashPlan, and I can only recommend it. You can decide to use a different tool, the ideas highlighted here will probably still apply. CrashPlan is composed by a multi-platform application (available for Mac, Windows, and Linux), and a storage service. The storage service isn't mandatory, a nice thing about CrashPlan is that you can use external hard disks, or other machines as backup destinations. Everything is encrypted, so you don't need to worry even if you backup to somebody else's machine. That being said, CrashPlan recently made available an unlimited plan, which is very affordable and makes thing even easier if you decide to use their storage service, called CrashPlan Central.
Last week I was on a business trip, and I had to prepare an urgent document. I wanted to start quickly, so I opened an old document just to re-use the template (I had not saved the template itself); I started writing, and after some time I completely forgot where I started from, so I started saving my work. I finished my document, and delivered it. It was only a couple of days after, when I was looking for the original document (the one I used as a template), that I realized it was gone... I had overwritten it while periodically saving the other one, and I was at 5,000 kilometers from my time-machine. I was almost to the point of restarting it from scratch, when I remembered CrashPlan. I opened the application, typed my password, and was able to choose from different versions of the same document, saved at different times (CrashPlan+ allows you to monitor the filesystem in real-time, and generate a new version of a file when it is changed). It took less than 30 seconds to retrieve the correct file, and this saved me some hours of work. Also very interesting, if you use CrashPlan Central, you can restore your data on a different machine directly from the web.
This is not the only use of having an online backup. This kind of services is also the easiest way to keep an off-site backup of your data, which is always up to date. Obviously, you need to pick a company that you trust with your data: they can claim to encrypt your data, not to be able to access your files, but I don't see how you can verify these claims. To me the advantages clearly outweigh the security concerns, and I do recommend this service without hesitation.