#1 rule to consider after determining a question may be a better fit elsewhere:
Don't migrate crap.
In going through the close queue, I often see questions voted to migrate, but the question needs improvement. This should be done before we migrate it, otherwise we've just decided it's a bad question for us and then sent a bad question to another site, which they then need to vote to close.
There's no reminder of the "no crap" rule when selecting to migrate, but it's something that we need to be better about. For those who don't have it, here are some stats:
So it's not just SU that has rejected migrations, but I think it's something we should be able to improve on. I bring this up because I know a lot of close-voters do frequent meta and may want to take a little bit more time to scrutinize a question in the close queue to determine whether it's a good question in the first place.
One other thing that some may not be aware of - migration results in a post score resetting to zero. Maybe use this as a guide.