one of the biggest ways they come in is by divers not dumping air when vertical.
With a bc you never have to raise the inflator over your shoulder which is a very uncommon thing in diving unfortunately.
most people do that. All that does is let water in the inflator the whole time and is completely unnecessary. All you have to do is raise it so it is parallel with the elbow to let all the air out. Hell most instructors do that
It won't let me insert the image here, but
http://img.diveadvisor.com/photos/bcd-6227.jpg
that is as high as the inflator should ever go. That will help with water getting into the bladder, but also save strain on your hoses, keeps you from looking like a damned moron in the water, and is actually the easiest way to ascent. If you keep the inflator at it's normal level, keep the oral inflation button depressed and slowly raise it to where the air starts coming out. On your whole ascent while vertical you keep the button depressed, and move the inflator up and down slightly to control your ascent rate. Because it is open, the wing will automatically compensate for the decreased pressure by equalizing the whole way up and you can control the ascent by slowly sculling your feet. It's the easiest/best/most efficient way to make a vertical ascent. Bit trickier when you're flat coming up, but still possible, just is a controlled lean to the right.
As far as draining the best method is as follows.
Inflate BC fully
hold the plate flat in your left arm with the inflator pointing to the left, butt dump closest to your right shoulder tipping it slightly so the dump is the lowest point in the wing
SLOWLY pinch the dump valve until it is cracked just enough that water is coming out. If you pull it all the way open you'll never get the air out. You need the pressure to push the water out. Once air does start coming out, reinflate and do it one more time and you will be about as dry as you'll ever get it.