Listen to this episode Backup your data before you upgrade to Mac OS X Leopard 10.5. In fact, even if you aren't upgrading to Leopard -- Backup Up Your Data!! Everyone uses their own solution, but here are a few recommendations. Some are free, some cost some $$, but each have their own set of unique features. Find a solution that works for you and fits your needs.
Full Drive Clones
First let's go with doing a total clone of your hard drive. To do this you will need another drive and/or partition that is approximately the same capacity as your OS X Drive. What's nice about this method is that if Leopard totally bombs your system, you can boot from your cloned disk with all of your original data & operating system. There are two great utilities each with their own set of features. But they both have one feature that works well, and that's making an exact bootable copy of your hard disk. The first program is SUPER DUPER from Shirt Pocket Software, and it's well worth the $27.95, due to the versatility and extra features it offers. Another alternative is Carbon Copy Cloner, and it's Donationware!! Both will duplicate your disk for free and area equally great programs.
Partial & Incremental
If you want an incremental backup, and you subscribe to .Mac, you already have the Backup program. It will allow you to copy files and folders and store them one another drive, your iDisk, or burn to Cds and DVDs. This is nice if you already have .Mac. A free solution however is a program I really like called iBackup. It's a another donationware program and it will backup all of your files and folders, and you can then burn them or copy them elsewhere. Personally, it's the program I prefer to use when I am backing up and not doing a full drive clone. There are a plethora of options, but these are a few quick suggestions that I have tried, and I like them all, depending on your needs. NOW GO BACKUP!!