This is from my experience and it hasn't failed me yet. It actually saved me in more ways that I can count:

1. Stay away from Dev modules(mod), unless you want to be part of the user testing.
2. Create a backup before you add the mod. Using Direct Admin works best for me.
3. Add the mod in the correct folder. " ........../domains/yourdomain.asu.edu/public_html/sites/all/modules" is recommended for Drupal 5+.
4. Read the readme.txt file that is with every mod to study changes and what it does to your install. (You will need to extract the compressed folder to read it.)
5. Also check the ReadMe.txt if the mod has any dependencies such as CCK, Views, or core modules.
6. When you login with your admin account, check "User Management >> Access Control" to see if you need to add mod use permissions.
7. Don't install any other module to monitor conflicts for at least a week. It is VERY hard to find a problem in Drupal with so many multiple variables.
8. If you don't like it, disable it first, then delete the folder in direct admin.
9. Check you see if their is another way, most likely someone created something else that works better for your needs.
10. If you find a mod that work great for you, tell us about it.