If you dont already have a 3rd party backup product i would install the 2 week trial version of either LiteSpeed or SQLBackup and use this to compress the backups to up to 90%.

I had the same issues moving files from one data centre to another and used Litespeed to compress the backups. An 80GB native file dropped to under 7GB and made transfer much easier using XCOPY in restartable mode (wont copy ACLs though whereas Robocopy will).

Using diffs and logs as Ninja suggested can also help the process for very large databases