Hi Serenity,
I find some assisted stretching integrated with massage can really help improve ROM and relieve pain. First, massage the neck & shoulders well to increase circulation & warm up tissue. Then, add the following techniques. These techniques focus on the neck and are all done in the supine position.
I use Reciprocal Inhibition: We all know that muscles operate in pairs; when one contracts, its opposing muscle relaxes. The idea is to induce the inhibition response prior to stretching the muscle, thereby relaxing it and increasing its flexibility. Support your client's head and passively move their head so their right ear is moving towards their right shoulder (stretching the left side of the neck). Ask your client to tell you the minute there's any discomfort. At that point, move the head back slightly, until there is no discomfort. Place your hand on the left side of their head and ask them to gently press into your hand, is if they were trying to move their head back to the central position. Tell them you will resist/prevent their movement. They should only use 25% of their strength, and hold that contraction for 5 seconds. Count for them. After 5 seconds, tell them to relax, move their head a little bit in the direction they were pressing and then repeat the stretch you started with: moving their right ear toward their right shoulder (lateral flexion). Ask them to tell you again when/if there's any discomfort. Usually, they'll already be able to stretch farther than the first time. I repeat this contract/relax/stretch 3 times to each side.
I then use the same technique for neck rotation. Support your client's head and passively turn their head to the left (as if they were trying to look to their left). Ask your client to tell you when there's any discomfort. Turn the head back slightly, until discomfort disappears. Place your hand on the right side of your client's head, and ask them to try and turn their head back to facing forward. Tell them you'll be resisting/preventing movement and again, they should only use 25% of their strength and hold that contraction for 5 seconds. Count for them. At the end of 5 seconds, tell them to relax, turn their head slightly back toward center and then repeat the original stretch (turning their head to the left). Repeat as needed, like above. And do this movement to both sides.
Another technique that can be very effective is Active Isolated Stretching, which works on the principle of reciprocal inhibition combined with active stretches (client does the stretch), but for only 2 seconds at a time (before any guarding happens).
Lastly, it's sometimes quite powerful to have the client activate their muscle while you massage it. For example, while you are doing trigger point therapy on the SCM, have your client rotate their head the opposite direction. While you are massaging the posterior neck, again have your client slowly rotate their head (only slightly this time) from side to side. You can also have them slightly laterally flex while you work the posterior neck, too.
I hope these ideas help. Good luck!