Omitting files from Time Machine Backup

In the interest of conserving all the space I can on my ext HD, could anyone clarify for me what MUST be included for an acceptable back up with Time Machines initials scan?

There are obvious programs and files I know I can omit, but I'm a not so familiar NOOB with Leopard and I'm apprehensive about possibly not including something crucial foundation of the back up.

Any help and suggestions for an 'outline' of what to save with TM would be greatly appreciated.

Grazi

HTAM

I'm of the opinion if you're going to create an effective backup strategy in order to recover in case of total HDD failure, an exact clone of the disk should be made. Purchase a larger external HDD and use it to make backups of the entire internal drive. Large external drives are inexpensive. Shop around.

If you want to continue using your current drive, Time Machine will never try to exceed the space limit of the drive as it will delete older backups to make way for the newer. Anyway, how valuable are old backups to you? By old, I mean a month or later.

Regards.