I "taught" my mare to move off of a tap on the base of the neck-so, if I wanted her to go right and she wasn't immediately responding to seat and leg, I would tap the left side of her neck with my finger. Before she got laid off, we had an awesome whoa-I only needed to sink my back down, push my feet forwards a little, and she'd stop dead. Bridleless stops are a little more shaky now (it takes her 4 or 5 steps instead of 1) 'cause I haven't been on her in a while, but I don't think it'll be too hard to get her back in form.
Anyways, I'm rambling...the point is, get your horse going perfectly with a bridle before you start considering without. As others have said, bridleless/tackless riding is just a by-product of good training. It's not generally something you specifically train for.