SQL Server 2005 - Map login to pre-existing user
For a failover-type situation, I use the handy 'sp_help_revlogin' stored procedure as published in http://support.microsoft.com/kb/918992

This procedure, when run on your primary server, will script out each login WITH the SID (this is the important part for making sure your logins are mapped to their respective database users) and hashed password (so the password remains the same on each server.) The resulting script can then be run against your failover server to create the logins. Be sure to review the output before running it.

As-is, it scripts all of the logins in the server, but could be modified to script only the logins you're interested in. It could also be automated to generate a script on a regular basis.