Make sure the tack fits PROPERLY.
Make sure you look at doing carrot stretches and under-saddle flexion exercises. Jane SaVoie has some GREAT ones. Look her up on her "Supplying" and "+7 +1" exercises.
THIRD and most importantly, do not force your horse into a frame or their head down or anything like that. This means do not just shorten the reins and pull your hands back and expect them to relax. Now if they're stiff as a plank, then try some of the flexing exercises. Start at the halt, then the walk, then the trot, then the canter.
Make sure you fully understand what you're doing. Don't just monkey-see monkey-do it because there is a feel to it and if you do it wrong then it does not help them out.
Best of luck :)
"Strength is the ability to use a muscle without tension"