After laying off of any running for over a week, and only doing cross-training (elliptical trainer and cross-ramp) I'm now ~85% healed from my right Achilles tendonitis!
With the benefit of hindsight I realize better the importance of a gentle stretch being incorporated into my regular routine - as well as to help recover from this condition. The type of stretch that is working best for me is to place both my feet on a 6" elevated platform, then while standing allowing my heels to drop naturally - thereby stretching the Achilles tendons. I work hard to not overly vigorously push down my heels, and I discontinue on incurring any pain. I've been successful with two repeats of two-minute stretches, done immediately following a non-impact cross-training work-out to warm-up my muscles / tendons and increase my blood flow. I also realize that in the initial day(s) following development of the tendinitis - where my walking gait is affected via a clear 'limp' and I feel the Achilles tendon pain I take an anti-inflammatory (Ibuprofen) and apply ice directly to the Achilles tendon in order to further reduce inflammation.
Bottom line that works for me is to NOT stop exercise - since a case of tendonitis might take two-weeks to fully heal and as cessation of exercise for this duration would severely compromise training and fitness. Instead I attempt to maintain my cardiovascular fitness via the elliptical trainer and/or the Precor 'CrossRamp' machine - or biking or swimming (though I'd worry more about degradation of running-specific fitness since swimming is more of an upper-body activity). Achilles tendonitis as just another challenge which with good treatment, and avoidance of the HUGE temptation to run on before the condition is fully healed - can be easily overcome.