What are the best stretches, exercises, and other preventative measures for tibialis posterior tendinitis?

Ankle Dorsiflexion: Ankle dorsiflexion means to move your ankle so your foot moves up towards your knee. This can be done while standing by putting your foot behind you and stretching your heel to the ground. Alternate sides. Go gently at first as you can actually over do stretching. Good luck!

PREVENTING: First, I congratulate you on using the correct terminology; tibialis posterior, not posterior tibial. Second, do you really want to stretch a tendon whose job it is to maintain the integrity of the arch; a TP tendon that lengthens only 2cm can cause pathology. Third, prevention. There are many causes; the most common may be an intrinsic abnormality within the tendon itself. Age, BMI, steroids also