I've talked about backing up your mac using different tools in the past, but what's equally important as a backup is the ability to restore it.
In the past I used to boot 1 machine normally and boot the one I wanted to restore in Target Disk Mode by holding the 'T' whilst booting. That way, the second machine would function as any external harddrive, which can then be accessed from the first Mac. I would then restore an image I created earlier using carbon copy cloner or something just like it.
Recently I had to buy a few of the new MacBooks. The nice unibody with black keys and glossy glass screen are gorgeous IMHO, but the lack of a FireWire port is definitely a problem. For one the lack of FW also means there's no Target Disk Mode. (I don't know why they can't implement it using USB, if you do, please enlighten me).
No Target Disk Mode unfortunately also means there is no easy way to re-image these machines the way I'm used to.
So, this is what you need to do:
You need to create a bootable external USB-drive (very cheap these days), boot from that on the MacBook by holding the 'Option'-key and selecting the bootdisk on screen, then restore from an image or from the external drive itself. (I prefer creating extra images on the USB-drive so I can always fall back to some known-good-state, or re-image a machine for different purposes/users quickly and easily)
It's cheaper than requiring 2 macs, but it wasn't something I expected, so I got caught once. Hope you can avoid it after having read this.