There's a couple things that can help you with this:
A good excercise is to Walk around with a tennis ball under your chin, and hold it to your neck without using your hands. This will help for tucking your chin naturally.
Another thing to do, practice keeping your hands up and mimic the parrying motion that Rampage uses in most fights. Do this excercise until your central shoulders exhaust. This builds your neck muscles to cushion impact on your chin and it also makes it muscle memory to keep your hands up.
And depending on how high your shoulders/trapezious raise up your neck naturally, you might wanna practice raising your shoulders in a relaxed manner. Tucking your chin to your shoulder so as to give complete protection from one side. Slight leaning can then deflect punches coming at your temple and the chin is still protected by the shoulder. The Bernard Hopkins/Rashad Evans video on this site goes into good detail about it.
Footwork excercises can also do wonders at masking this weakness. If you can move really well in and out, sometimes the punches just don't even reach you.
But none of that is really going to help unless consciously you are cognizant of how high your chin is raised. You never want it too upright, without protection and without both head and body movement.
BJ Penn beat Frankie Edgar more times than Benson Henderson beat Frankie Edgar.