I agree that this can be an issue but you can help yourself out a little by reviewing the syntax of the "CREATE LOGIN" or "sp_addlogin" commands.

Both of these commands allow you to specify the SID. If you are able to specify the same SID for the logins on each of the servers, you can happily backup/restore databases between instances and the login and user entries will match because you created the login on each instance with the same SID.

This is especially important when you are using log shipping or database mirroring. When a problem occurs and your primary server dies, there is one less issue that needs to be handled to get your standby database online. In the case of mirroring, ensuring that you use the same SID allows you to automatically bring the mirroring online with no intervention required.