Best practices before doing any Mac OS upgrade:
0) Read the info on compatibility with your software:
Mac OS X v10.6: About incompatible software - http://support.apple.com/kb/HT3258?viewlocale=en_US
1) Always backup your data.
Make a good and complete backup with Time Machine and/or make a copy of your disk with Disk Utility.
How to back up and restore your files - http://support.apple.com/kb/HT1553
Time Machine 101 - http://support.apple.com/kb/HT1427
2) Understand how to restore your backup (you did make that backup!) before you start.
Recovering your entire system - http://docs.info.apple.com/article.html?path=Mac/10.5/en/15638.html
How to back up and restore your files - http://support.apple.com/kb/HT1553
3) Make sure that all software updates have been completed before upgrading.
4) Repair the startup disk with Disk Utility before upgrading and after upgrading.
Boot from the install media and upgrade.
