Like most people, I use a set of ratcheting straps and work my way down the boat, tightening the hull to the deck, then taping them together using strapping tape. Working a few inches at a time and prying the hull out (with a few putty knives) to mate with the deck before you tighten the straps, the ratcheting straps should apply enough pressure to keep the edge of the deck and the hull together where they meet.
This will not work if you've tacked the fiberglass tape for the inside seam of the hull in advance, so you may have to undo that for the sections where the hull and deck do not mate up easily.
Once the fiberglass tape cures on the inside, it will hold the boat together when you remove the strapping tape and glass the outside seam.