Carrot Stretches are a good idea :) Here's a vid that will help you :
When on the horse, do a TON of circles and bending, Figure 8's and Serpentines.
When I get on Nelson, I do at least 15 - 20 minutes of walk work. The walk has to be a working walk though, where he's opened up front, moving under himself and stretching. Then I merge into Lateral Work, riding inside leg into outside rein.
When the neck muscles are lax, and his jaw isn't tight anymore, I move into the trot.