I have been seeing a chiropractor who also does acupuncture, and that has helped loosen my neck up quite a bit. He uses massage, acupuncture, and/or adjustments depending on how stiff my neck is on that particular day.

I also do a series of neck stretches every day (in the shower so the muscles are warmed up a bit): chin to chest, eyes to sky, ear to shoulder, rotate to each side, and chin tuck. I do 5 sets of these, holding each stretch for 5 seconds.