On the neck near where the top of the fret board is going to be....on the right hand side near the edge ....drive in one of those tiny skinny nails or brads or whatever they call them ...but only about 1/2 way....down at the bottom of where the fret board is going to be on the left hand side ...near the edge drive in another one...only 1/2 way in.
Now take wire cutters and on a angle (so the tops of the little nails end up sharp) cut the little nails.
Now apply the glue to the neck and fret board in whatever manner pleases you.
Now take the fret board and place it over the neck in the position you want it to be.
Now press down so the sharp end of the little brads goes into the fret board
Waaalaaa now it is going nowhere....clamp it and no worries about slipping.
I like to do a dry run. Clamp the fretboard to the neck. Then I drill a couple of small pilot holes where the fret markers will be. Then drive a small nail through the fretboard into the neck using the pilot holes. Disassemble, glue it up, push the nail back through the fretboard into the neck, and clamp. I still check it about 10 times to make sure everything's good.