Yup, overhead casting and mend, mend, and mend some more is the way to keep your bait or jig a head of your float where its supposed to be. One thing that wasn't mentioned (if not obvious) is that the surface currents pushing the float is slightly faster than currents at depth, so the float will travel faster than your bait if not mended to keep the main line dragging behind to slow it down a touch.

Another tip is, when possible, situate yourself upstream of the fish and cast straight out instead of upstream. Casting straight out and mending as soon as the line hits the water gets your bait optimal in a hurry, and if you're already upstream of the fish....