Work on keeping your knees and feet shoulder width, this maybe hard at first depending on the length of your legs. Your knees or feet may have a tendency to turn in or out. Symmetry is a very important concept in understanding alignment and in this case misalignment could stress the knees. Keep your weight on the lower abdomen. Many students tend to shift their weight forward onto their hips and other students lift up onto their hip points to try and kick the legs higher. Staying on the lower abdomen will more effectively work the areas of the body needed to maximize your results.
Look in the mirror and see your shoulders, elbows, wrists and knuckles are all shoulder width. The wrists have a tendency to protrude out to the side which actually inhibits the full extension of the legs. To correct, simply rotate your hands so the knuckles turn up toward the ceiling creating a straight line from your elbows to your knuckles. Keep your legs and feet shoulder width to access front of the hips (Hip Flexors). There is an inclination to avoid this stretch by allowing the legs to go outside shoulder width. Don’t let that happen!
Once you can fully extend your legs way past your head begin to internally rotate your elbows, (toward each other) and relax your head all the way back. This shoulder rotation will permit your middle spine to bend allowing you to go much deeper in the pose.
The Full Bow or Poorna Dhanurasana is not the full expression of the Bow pose. It is an entirely different posture. However, unless you have an extremely flexible spine the Full Bow also requires significant shoulder flexibility, specifically the anterior or inner deltoids. It will be very difficult to attempt this advanced pose without guidance, so I suggest you find one of our certified Level II instructors for help.