That's the trickiest place to keep from leaking...somewhere along the line I read a post (not sure if it was from here) about bolting in sections, kinda like torquing head bolts down. They recommended doing the upper vertical cab bolts first to suck the thing all the way forward and make sure you get a good seal all around the cab, then backing off on all the cab bolts and tightening the two side rails right next to the cab to get those front corners nice and tight. After that you can tighten up all the cab bolts and finish all the side rails. The factory also sticks a little ball of tacky sealer up in the seam where the map light harness pokes out too so if that's left open it probably doesn't help.
I dunno if it actually makes that much of a difference but mine has been off and on many times and hasn't leaked yet. I also wet all the seals with silicone spray just before reassembly and I believe that helps as well. FWIW my seals all "look" worn out but I've yet to see a drop in the cab.