I wouldn't put a regular distro on a flash drive. A flash drive has a limited number of writes before it won't write to that sector any longer. The common number quoted is 100,000 writes, which seems like quite a few, but it really isn't. I'd find a distro built specifically for a flash drive. Puppy linux, or Damn Small Linux, or even a custom Fedora spin would work, but make sure they specify that it's for a flash drive.