People often times say bow your head when they prepare to pray. This is not required or even always appropriate.
If you feel ashamed and are asking forgiveness, then this would probably being an appropriate posture to take. But what about when you are excited or nervous or seeking guidance? There are any number of situations where it is more appropriate to look up as a child to his father.
When I talk to God I talk to my Father and am not afraid, but rather expectant. I acknowledge my failings and flaws, but I look to Him for love, not condemnation. God loves me just as I am even though I don't deserve it or completely understand how he could. I am His son and I look forward to time spent with Him.
I know I don't talk to him enough, but when I do, it would be like any other son and father relationship. Sometimes I am ashamed and sometimes I am proud. Sometimes I am sad and sometimes happy. Whether I look down or look up I know that He still loves me and that makes all the difference in the world.