Given that the bulk of the time taken to do a backup is I/O, the things to do to speed up the backup are based around reducing that - use whatever in-built compressions you have within SQL Server to compress the data files and the backup files being created.

The other thing you can do is make sure that your database isn't cluttered up with data that's not needed - look into various archiving strategies for really old un-touched data that you only need for compliance / audit, for example. Archive it off into a separate database, or keep an "old stuff" filegroup that doesn't get backed up as often as the "current stuff" filegroup. Hmm. I'm going to have to think about that one...