This one came up at a client yesterday. It is a feature that we take for granted, but realize not everyone knows about it. It can be a lifesaver when you find you saved your file with a mistake in it and wish you could go back to the prior version.
By using the Save As feature, you can automatically have a backup saved with each save. The backup version is one version back from the latest version you saved.
Suppose you saved a file at 9 AM. Then say you save it again at 9:10 AM. The version you saved at 9 AM now becomes your backup version.
This doesn't happen automatically. You need to set this up when you first save the file. Then it will happen automatically with each save.
Unfortunately, it is not intuitive. However it is simple. Here is how you do it.
Open your file or start a new file. Under the File menu, click Save As. Now don't click Save yet. Go to the top right of the the Save As dialog box and click General Options. (This is how it looks under the XP version of Excel).
This will take you to the Save Options dialog box. Check the box that says Always Create Backup.
The backup file will be saved in the same directory as the current file, but will have a different extension than *.xls. In case you need to go to the backup version, you can open it up in Excel.
You may not need this often, but if you ever do, this small step up front can save you a lot of backtracking.