I have a MacBook Pro with Snow Leopard installed. I skipped Lion because it seemed somewhat underbaked when it came out. My hope is that Mountain Lion will have fixed most of Lion's difficulties. I decided to do this upgrade carefully. I have a Time Machine backup, but I cannot use it to boot the computer. Also the disk is close to full, so I am always deleting old backups to add more files.
As I set about to make a bootable backup, I decided to move my Time Machine backup to a new larger disk. I found this article on Apple's support website. It turns out that I needed to convert my new Seagate 1TB USB drive from a NTFS volume with a master boot record to a Mac Extended filesystern with a GUID partition record. I did this using Disk Utility. The directions are available here. The copy of the Time Machine backups took quite a awhile to complete, but they did so successfully. I then swithced the Time Machine preferences to use the new disk and did a backup.
I reformatted the old Toshiba 400 GB disk that had been my Time Machine backup as Mac Extended filesystern with a GUID partition record. This is need to book the Macbook Pro with that disk. I then downloaded SuperDuper and cloned my internal disk to the 400 GB disk.