A few things:
most yoga classes I take, vinyasa and hatha, leave students in savasana for just a few minutes. The avg class length is 60 minutes, and according to part 5 of the savasana description, one should stay in the pose for 5 min per every 30 min of practice. Thus a 10 min savasana should be the norm, but this is seldom the case. Wassup wit dat?

On a different note, when I stay in savasana for an extended amount of time, I sometimes feel a heavy pressure on the back of my head where it meets the floor or mat. After coming out of the pose I sometimes experience blurred vision for 10-20 minutes thereafter. I wonder if a prolonged savasana puts undue pressure on the back of the head, near the occipital lobe, the part your brain that regulates site. Is this dangerous? Should I look to support my head more with a blanket rolled under the neck and head?

This site is a great resource from the perspective of images and classification of asanas.

However, the instructions are REALLY REALLY badly written. Hard to make sense of most of them. Either consider putting up videos for each asana or write clearer instructions.

All these instructions may distract the subject from the essential simple physical and mental surrender needed for shavasana. Better instructions: lie down on the back in such a way as to insure complete comfort. Use pillows if needed to achieve comfort and prevent pain. Let go of body and mind. Practice for one to two minutes (without referring to clock or watch) at the end of asanas, then resume daily activities.

I have found that complete surrender is easy when one practices a deep meditation technique regularly, such as Transcendental Meditation (TM) or Natural Stress Relief (NSR). Other forms of meditation may also enhance the effectiveness of shavasana if they are practiced completely without mental effort.

My lower back really hurts when I do it with the legs extended, and no true relaxation is possible with knees bent... I'm stuck on this

This really helped capture the posturing I was trying to attain for a true savasana experience. Thank you for taking the time to post this.