Backup contains only used pages (If it is full backup). No use of shrinking the database.

If it is a differential backup, it contains all pages changed since last Full backup.

So sometimes if your transaction is heavy, you can consider full backup itself.

As Lynn mentioned consider striping the backup .

Backup database databasename to

disk='c:\backup\filename1.bak',

disk='c:\backup\filename2.bak',

disk='c:\backup\filename3.bak'

Remember you will need all of these backup files to do a successfull restore.