One very effective approach would be to "partition" the table but that's only if you have the Enterprise Edition and, IIRC, if you don't mind the "archive" being in the same database. What that will do for you is make it almost instant to "archive" a previous month and it will reduce the nightly maintenance because you won't have to rebuild or reorganize the idexes for the entire table because, if you do it right, the indexes are partitioned, as well.
--Jeff Moden
Change is inevitable... Change for the better is not.