To streamline the process, download the free Lion DiscMaker app.
If you already own a copy of OS X Lion, you should be able to download a fresh copy at any time from the Purchases tab of the Mac App Store. Once the OS has finished downloading (and you've verified that Lion has landed safely in your Applications folder), you're ready to transfer it to your trusty USB flash drive. During testing it took me about 50 minutes to redownload the 4GB or so of content that comprises Lion. Click the Download button next to Lion, note the directory that it downloads to and wait for the download to finish. From there, you should see OS X Lion as one of your previous purchases. Go into the App Store application and open the Purchases tab at the top of the screen. This is a simple but slightly time-consuming process.
