What is the best treatment for posterior tibial tendonitis?

Exercising with orthotics and braces is the best non-surgical treatment for most individuals.

The first approach is to reduce or altogether eliminate activities that aggravate the pain. It is beneficial to switch to low-impact activity. Biking, elliptical machines, and swimming do not place a high impact load on the foot, and most patients handle them well.

Pain and inflammation are reduced by drugs like ibuprofen and naproxen. Limiting inflammation surrounding the tendon by taking such medications approximately a half-hour before a physical activity helps.

To keep swelling down, use cold packs to the most painful part of the posterior tibial tendon for 20 minutes at a time, 3 or 4 times a day. Avoid putting ice directly on your skin to avoid a cold burn. Immediate application of ice to the tendon following completion of an exercise helps to reduce inflammation around the tendon.

Doing stretches, strengthening and range of motion exercises helps strengthen and ease tension in the tendon facilitating better outcomes and healing.

I expounded on the exercises in this answer I gave just a while ago