Prospecting is labor-intensive key to generating strikes when there don't seem to be any fish around. The technique is simple: Take a rod rigged with a naked ballyhoo and freespool it next to your dredge. After it sinks well behind the dredge, pull it back up. If there's a fish hanging deep, waiting for a baitfish to stray from the "school" and become easy pickings, it should attack your 'hoo.
Turning on a strike is how you change single hookups into multiples. When a billfish (particularly sailfish and white marlin, which can aggregate together) takes a bait, initiate a turn toward the same side it appeared on. If it strikes a port line, for example, make a port turn. As you turn, your crewmembers should clear that side of the boat for the fight. One angler should grab a rod on the starboard side, then jig it, drop the bait back, reel it up again, and keep it moving and erratic. With the lines, dredges, and teasers cleared from port side of the boat, any fish shadowing the spread or following the hooked fish should be attracted to the starboard side of the boat-and that one active bait.
Changing is also key, when fish appear but don't attack. If a billfish "window shops" the baits then leaves, chances are there's something wrong with your offerings. Check every bait and teaser, to make sure they're weed-free and swimming properly.