What's the most effective way to treat achilles tendonitis?

Rest: Rest, ice, compression, elevation and limited use. Avoid running and jumping activities. In some cases a steroid injection may be performed, but may increase the chance of Achilles rupture if not done properly.

Rest stretch, repeat: Achilles tendinitis is best treated by rest from aggravating activities, use of a layered heel lift, and stretching exercises combined with anti inflammatories. The treatment will not result in relief quickly. The regimen should be religiously followed for the first 6 weeks, and repeated until symptoms are diminished!