So Mac OS X El Capitan -Mac OS X 10.11 - is just around the corner so here are some tips and tricks to help you complete the upgrade smoothly.
Tip #1 - Always (ALWAYS) have a full backup of your OS and important files before completing the Upgrade.
We recommend you complete a full Time Machine backup, and also have a Clone of your current OS.
If you use the wonderful Carbon Copy Cloner, you will need to make sure you are running 4.1.4 for it to work in 10.11.
The advantage of a Clone is that you can boot from it and get back to a working state if the worst happens or if some vital piece of software stops working after the upgrade. A Time Machine backup often updates before you have a chance to test everything. The OS may upgrade library files such as the Mail or Photos folders to later versions which may not be compatible on an older version of Mac OS. We recommend unplugging your time machine backup disk before applying the upgrade; and plugging it back in after you are happy that everything is working. You may need to erase and restore to go down to your previous OS.