Best practice for archiving old data?

We are migrating 3 slightly different SQL databases into one (SQL Server 2008R2). We want to keep old databases "alive" for some period of time but once we've gone live we don’t want staff accidentally going into old database, so taking the database offline for awhile then drop it would be the solution? taking backups for it and test it in another serve prior to do this of course.

I’m not sure what best practice is for making archives, but suspect that making SQL server backups and placing on tape may be a solution. Do you know if SQL Server backups are compressed and can be saved to tape? There may be better solutions than tape these days, but I think tape is still the standard.

Thanks for any advice