In addition to having it for at least 60 days, some registrars might require some time before it expires as well, then you get stuck with having to renew it if you wait too long.
Also, as mentioned on the Dreamhost transfer page, be sure that you unlock your domain first (if you haven't already), or the transfer will fail.
Seems pretty basic, but it can be easy to forget--or some people don't even know there is a lock, or that their registrar defaults to locked.
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