Before updating to lion
Another step to consider is to run a few maintenance routines on your hard drive before installing.
At the very basics, reboot the system into Safe Mode and then use Disk Utility to check the hard drive for errors.
Also keep in mind the App Store will check for system compatibility, so if you can download it to your system without error, then it should install just fine.
Next, ensure your system is not experiencing any major problems, which include regular freezes or crashes of applications or the entire system, or the inability to log in to a user account, enable sharing services, or establish hardware connections to peripheral devices or networks.
That's one reason why I highly recommend backing up your current system before you start the upgrade process.