We have received alot of questions regarding this posture... it's a goodie!
A few tips and tricks...
As you enter the posture, stretch up as much as you can and keep your arms with your ears. When you step forward try to keep your standing leg straight and thigh contracted...this will keep you straight and strong, it will also help you balance. As you come down into the posture, keep every muscle contracted... feel your body working.
The trick is to stretch your arms forward as you stretch your toes back, it's like a tug of war.
Stretch, stretch, stretch!