1) Let it warm up when you first put it into broadcast mode before exercising. It seems it takes a minute or two to calibrate properly - and during that time it gives it a chance to have a durable link to the paired ANT+ device (in my case, the 920XT). Not sure if this is necessary, but I got more satisfactory results this run (no disconnect from the 920XT or funky readings in the first couple of miles.
2) Wear the band right above your wrist bone, but not too high on your arm.
3) Find a comfortable tension for the band for your non-exercise time. Something tight enough where it isn't sliding around on your arm like a bangle, but not so tight that its noticeably tight. This is enough for good resting HR readings most of the time.
4) For exercise, tighten the band up a notch. Enough to feel snug, not so much you are tourniqueting yourself. Enough so when you loosen it up after exercise and move the band, you leave a light imprint of the stay loop on your arm (see the imprint above the strap on the top right of the band). This technique definitely keeps the HRM more "locked in" during exercise.