Depending on where you are going, the best way I can think of is to go to an Internet cafe once in a while and upload your pics to a web server. Or for a more low-tech solution, burn them to a DVD and mail the disc home.
My reasoning is this: You don't really want your backups to be travelling with you. Suppose your bag gets stolen; you've now lost your camera, memory cards, and
the backup. Last year a mate of mine was travelling through South America. He used a few SD cards, and somehow lost them/had them stolen, and came back to Australia with hardly any photos. Even if he had a backup, it wouldn't have helped him, since it would have been in the same backpack.
Travel with 2-3 memory cards. This will hopefully give you some time between uploads.
However, if really all you want it to put your photos so you aren't always running out of space on your memory card, you can buy external hard drives that have integrated card readers and will automatically copy files from memory cards. eg:
http://www.cdrinfo.com/sections/news/de ... wsid=16744