How can I put a Lion installer on a Flash Drive?
Your best bet is to follow the instructions on this page.

Things you'll need:


Flash drive bigger than 4 GB (8 GB+ would be best)
InstallESD.dmg (extracted from the OS X Lion App, if you've saved it prior to the upgrade)


There's an alternate method mentioned here.

Note: If you don't have the  Lion Install App, follow the instructions on this page to download it again.