Migrating to new domain name

But avoiding duplicate content problems

[GordonS]

How would you migrate your site to a new domain name whilst retaining your search engine rankings for the existing name?

We have a well-indexed site, with a poor domain name. We want to revamp the brand by offering the same site through a far superior name, and launch affiliate schemes on the new name.

We don't care too much about the new site being indexed at all. But we do want to offer the same site through both names whilst we migrate and we definitely don't want to scupper our hard-earned rankings for the old domain name.

We're concerned that doing this will wipe out both of our domains in the serps. How do we approach this?

(If necessary, we are able to use different "templates" for each site, although the content would be the same.)