put the foot your about to bring towards your hands straight up and back behind you first (
http://www.yogajournal.com/basics/2338), then bring through to hands. It should come further This is a trick I was taught which really helps me on those still days.
Also with down dog try 1/2 down dog on a wall. Focus on bringing heart down
www.ehow.com/video_4944898_yoga-downward-facing-dog-wall.htmlthat will help you feel what a down dog should feel like. Then when you go to do a full down dog bend your knees a lot to get a better shape in the upper back. It's first about the shoulders and back opening then you can start to pedal your feet up and down (lift and raise heels) to slowly stretch out the hamstrings.
It takes me 10 sun salutations to get to my normal state of flexibility.
the most important thing too is to breathe.....sounds simple but everyone holds their breath when the first start yoga (I still do it at times)