From your first class of ashtanga yoga Downward Facing Dog is the pose you do most often. You simply have to learn to relax there. It is the ultimate resting pose in ashtanga yoga. That's why it's important to do the pose correctly, not only to avoid injury, but also to make it as comfortable as possible.
There are numerous instructions on the internet how to do Downward Facing Dog and I will not write that here. I want just to say that this pose is all about upper-body strength. If your arms and shoulders are weak, you might turn your shoulders up to your ears. This is a big no-no, as it can cause injuries. Be sure to create space around neck. If you find your hands shaking or shoulders tensing up, it probably means you need to take a break.
A good amount of body weight is in your hands, so be sure this base is strong and stable. Don't lift up the palms... Make sure your back stays long and straight. If your hamstrings are extremely tight and you struggle to straighten them, you'll make mistake by rounding your spine. The best thing to do is to bend knees softly, so you can lengthen pelvis away from shoulders.
Yoga Teachers Training - Maui, October 2014
Week 4 of my daily yoga practice is started. Tomorrow is next practice.