Protein is what the body uses to grow your muscles, but I am not sure how effective it is in relieving soreness.
I find that green juice is a great for soreness, and works as well after the fact as drinking some beforehand.
Some high quality fat, like from avocado, is also helpful in relieving muscle soreness. The fat helps to bind up toxins that are being released, which can be contributing to the soreness. You can read the truth about fats at my hub about it. http://hubpages.com/health/Fat-The-Myths--The-Trut...
Valerian root is wonderful for relaxing the muscles, which can help relieve the soreness.
And of course, a heating pad or hot water bottle can help relax the muscles, while a cold pack can reduce the swelling.