Downwind sailing is usually a bit of a drag. Flapping sails, a lot slower progress that upwind, and even worse is that not feeling any wind on your face gives the feel that the boat is standing still. Can't do much about apparent wind versus real wind. We don't have a spinnaker, but I found that the best help for gaining speed in a downwind situation is using a whisker pole!
A whisker pole is a light, aluminum pole (ours is stored in the v-berth) that can be attached to the mast and the clew of the headsail (Genoa). The whisker pole allows the sail to keep better shape in light wind usually when running wing and wing (mainsail to one side and Genoa to the other)
A couple tips I got from the Forspar website that I didn't do in my first attempt shown above to help keep the best sail shape: Use a topping lift. Attach a spare halyard to the end of the pole attached to the sail to keep the pole's weight off the sail. Next move the jib sheet block as far forward as possible. If you can get it directly under the clew (bottom corner) of the sail you've gone far enough. Making this line a sharp angle to the bottom of the sail and fairly tight will then stop the sail from "skying" or raising up in puffs of air once again improving sail shape.
The Forspar site made a comment about the position of the whisker pole that isn't possible on my boat. "The pole should be flown level (Thus the need for mast track and cars!) and at the same relative angle to the wind as the main boom" Training wheels has a stationary whisker pole attachment so that the pole shouldn't be level.