Drop sets and down-the-racks are ideal for blasting the deltoids. Down-the-racks were a favorite method of Larry Scott, probably the best small-boned bodybuilder to ever develop delts as massive as those of the big boys.
Deltoid exercises are particularly suited to down-the-rack training. To perform down-the-rack sets, simply warm up for a few sets and then go to the heaviest bells you can get six reps with. As soon as you fail, be it five, six or seven reps, immediately return those bells to the rack and grab the next lighter pair, which will probably be five pounds lighter.
Don't take any rest. Do as many reps as you can and then return those dumbells to the rack, again immediately grabbing the next lighter bells and so on.