What instruments do you use?
You want to use the same basic instruments that you do upwind-boatspeed and true wind speed-plus the apparent wind angle. You need to watch the latter because your optimum wind angle downwind is a little harder to feel by the seat of the pants. When you're beating, there's a very narrowly defined apparent wind angle where the boat will feel right. But on a run, your wind can be anywhere from 60 apparent to 180 apparent to make your best velocity made good (VMG)- depending on the wind and waves. For example, in 10 knots of true wind, your best VMG upwind might be obtained somewhere in a groove that's about five degrees wide (from around 26 to 31 degrees apparent). Downwind, however, the envelope that includes your best VMG is much bigger-it might be 25 degrees. So a good helmsman is keeping track of boatspeed, true windspeed, apparent wind direction and VMG (from an instrument, if available, or from a boat performance chart).

What can you do if you don't have the fancy instruments?
You have to keep watching how you're doing against the other boats. They are your best indicator of relative speed, and you should keep experimenting. On a run, for example, jibe more than you think is right (it won't slow you down too much), and how you're netting out with the boats around you. I think it's great to learn how to sail by the seat of the pants and not get too dependent on instruments.

At night, one of the best guides you can have is a lighted Windex at the top of your mast. You don't want to steer looking only at the Windex, but you should be able to use it as a reference. I once did a 1,000-mile Mexican race with Dennis Conner and we had a run all the way in a lot of breeze. Dennis didn't care about the Windex and didn't care about the instruments; he shone a flashlight on the telltale on the windward shroud and steered almost completely by that to avoid sailing by the lee. You'll get a pretty stiff neck you keep looking up at that Windex all the time. Another good place to put telltale is on the backstay over your head.
Like upwind, having a good "feel for the boat and the wind can be very helpful. The good guys kind of feel the breeze on the back of their necks, which really helps them anticipate what is going to happen. Did you ever notice how short Conner gets his hair cut before big race? He has it trimmed very close in the back, and I hardly ever see him look behind downwind.

Another way to keep track of everything that's going on is to get some help from your crew. Don't try to do everything yourself. Designate one person as the tactician, and ask him or her to keep an eye out for the little puffs and how the other boats are doing, and to keep the communication going with the trimmers. Make sure your tactician reports what's happening behind your back, because you'll lose your concentration if you have to keep looking back over your shoulder to see what's going on.

What's the optimal amount of helm when you're steering downwind?
On the run, getting the right amount of helm is just as important as it is upwind, except that your goal is to balance the helm so that it is neutral. In other words, you should minimize the amount that you have to turn the rudder. Having helm downwind is definitely slow because you just want the boat to go straight. You're not asking it to have lift or to go any closer to the wind, so all you're doing by turning the rudder is creating drag.
Moving crew weight around is one of the best ways to balance the helm. If you have leeward helm, move some or all of the crew to leeward; if you have windward helm, get them up to windward. The helmsman is definitely the key guy in terms of feeling how much helm there is and what needs to happen to balance it. Balancing the helm on a reach is a different story. Even though weather helm is slow, sail area is fast when you're reaching. Therefore, it pays to put up as much sail as you can control, even though this makes the boat heel over and develop helm. While you'd never sail upwind with more than four or five degrees of weather helm, on a reach you may be fastest with upwards of 10 degrees of helm.

Is there a "groove" downwind like there is upwind?
There is definitely a groove where the trimmer feels like he's got some pressure on the spinnaker and the driver feels like the boat is going well. You just have to experiment to find the angle where the boat feels lively and the sheet is pulling, and try it for a while. Watch how you're netting out against the boats around you. The best drivers can feel a little pressure on the helm once they start cooking; it's kind of a vibration. Again, the speedo is the key instrument, and the helmsman essentially lives and dies by it. Figure out what speed seems to net out well, and then steer the boat up and down to maintain that speed.

What most people have the hardest time comprehending is the necessary interrelation between the spinnaker trimmer and the helmsman. Finding the groove requires communication between these two more than anything else. The spinnaker trimmer should be saying things like, "It feels right here, good pressure. Come up a little now because I'm losing pressure, or come down a little because I'm gaining pressure."