(1) Do exercises to relax the neck, for example: http://my.yoga-vidya.org/video/relax-your-shoulders-with-yoga-1
(2) Strengthen you neck muscles. I only have a German Video on this - you might want to just watch the visuals: http://mein.yoga-vidya.de/video/nacken-und-schultern-staerken The principle: You push gently with your head against the hands for about 10 seconds. You do this every 2-3 days. Watch the photograps on http://www.yoga-vidya.de/de/asana/schulternacken.html
(3) You change the main Asanas in a way that you can do them. You do dog instead of headstand. You do supported shoulderstand instead of normal shoulderstand. In fish, you put the back of your head on the floow instead of the top of you head. In inclined plane and Cobra you keep head and neck straight (instead bending to the back).
(4) You train yourself to deal with stress