In my opinion, any physically stored data is more or less doomed. At some point in time it will be lost, damaged or flawed. Probably your best bet is redundant online storage if you want to be double certain you don't lose anything ever - Upload your photos to two photo storage sites. Either in something like Picasa and/or Flickr. Or in a non-photocentric service like dropbox. I personally use Picasa because the client makes it just one click to upload all my photos.

If you insist on a physical backup, then it's either SD cards, thumbdrives, portable HDs, or discs. I guess it depends on if you want to carry the backup with you (might defeat the purpose) or mail it home. I don't know your situation, but I wouldn't delete your local copy until you can confirm that the mailed copy made it safely. The mail system in many countries is very, "individual results may vary".