solid FOUNDATION, hands and arms in this case, is the first element to focus on. every single hand and arm muscle needs to be engaged: fingers, palms, forearms, upper arms, the whole lot, otherwise the whole body weight presses down on the wrist joint = not good. with the hands strong and finger pads pressing towards the ground, the weight will be instead distributed throughout the palms and fingers.
next, a bit of ROLLING IN AND OUT action to keep the shoulders and upper back wide: forearms are rolling in = towards each other; upper arms are rolling out = away from each other. this helps the shoulder blades to widen, flatten on the back and move down the towards the tailbone... it's like a domino effect up and down the body.
UPDATE: an important note on the sequence of actions of the arms: first of all, the top of the armbones need to pull back into the shoulder sockets to fully integrate (armpits should feel hollow as a result); next, the biceps (upper arms) roll out to widen the shoulders; and finally, the forearms roll in a little bit to balance the action of the upper arms.
with the foundation sorted, lengthen your tailbone and lift the sitting bones towards the ceiling, in other words: STICK YOUR BUM UP IN THE AIR. yes, that's a technical term :) if the bum is not sticking out, the lower back gets strained. this is one of the key alignments of this pose and the best way to achieve it is: with the knees still slightly bent, roll the inner thighs back and out (hope this makes sense!). keeping this, it is then safe to start straightening the legs without hurting the lower back. and finally, extend the heels towards the ground.
completely straight legs (without locked knees) and heels all the way down on the floor is the ideal (don't confuse with optimal) version of the pose but, with tight hamstrings being quite the norm these days, it is absolutely enough and ok to IMAGINE the knees straightening and the heels moving towards the ground rather than actually touching it. SAFETY FIRST, you don't want a sore lower back or any other part of you.
enjoy the resting pose!
p.s. you can find a brilliant anatomical study of the pose here, scroll down to adho mukha svanasana.