Before you upgrade to Lion, best practice recommends that you take the following steps:
1) Confirm that your Mac is Lion capable. (For this, see the previous blog post.)
2) Make sure that you have a full backup of your current Mac. This backup could be a Time Machine backup that is up-t0-date or a clone of your current Mac. Good tools to make a clone of your Mac are SuperDuper or Carbon Copy Cloner.
3) If you are running Mac OS X 10.6 Snow Leopard, make sure that you have upgraded to the latest version of the OS (10.6.8).
4) Do an inventory of your applications and printer(s) to make sure that all components are Lion compatible. You don't want to install Lion and then discover that an application you absolutely require - Quicken 2007, for example - will not work under Lion.
5) Use Disk Utility to confirm the condition of your hard drive prior to the Lion installation.
6) Make sure you have an Apple ID and are connected to the App Store to purchase Lion.