This is such a painfully obvious thing, but I've seen people recently who hadn't picked up on it so I figured I'd just throw t out there.
So, when you are priming a model in multiple pieces, mask off the bits that you are going to be dabbing glue on. Once the paint is dry, you can pull off the tape and you've got naked plastic ready for adhesemintizingination.
Simple? Yeah. Obvious? Youbetcha.
Such a basic concept that it's incredibly easy to forget?