What's the best technique for applying adhesive to the underside of ceiling joists?
Not much to it, really. I glued and screwed ~1000 square feet of type X to 3" strapping and I don't think any "dripped off."

Standard "cut tube at an angle and hold gun at an angle so the stuff smooshes out and sticks" technique. Pretty much the same as any construction adhesive. 

If you are getting a round bead that falls off you are doing it wrong - the applied bead should be about 3 times wider than it is thick, as a guesstimate. The short end of the cut tip should be in contact with the surface as you dispense, the long/pointy end should be slightly off the surface, and speed of movement and dispensing are adjusted to maintain the correct bead shape.