Oh, this is something I can talk about all day, because it's the single most horrible exercise in the whole techreational world to me
Keep your eyes OPEN!!!! There is no law that says you can't, and it's amazing how much information you can gather, even if things aren't sharp.
Touch contact -- If you're the guiding diver, make sure your contact and your motions are clear and unambiguous. Turn the thumb up and move the hand up and down to make sure the other diver knows to ascend. Give the "level off" signal (hand moving back and forth on a horizontal plane) WELL before the depth, so the maskless diver has time to receive and process the signal and begin dumping to stop. As Richard says, hold the RIGHT hand, or you hamper the other diver's efforts to control dump valve and wing.
If you are the maskless diver, try to relax. Stay in trim; getting out of trim really makes it more difficult to control buoyancy with precision. Use your teammates as a visual reference -- with your eyes open, you really can do this. Monitor your breathing carefully, as it's easy to get a little anxious and therefore a little positive.
I handle it pretty well with my eyes open, but I hate opening them to 44 degree water, so sometimes I forget to do it, and then all you-know-what breaks loose