While still at full draw push your string hand elbow down against your side. Just press it to your rib cage and push in. You'll end up with your forearm and bicept pushed together. Your hand will still be in contact with your cheek. It sounds odd... but it does work
This is exactly what I've done and it does work. Back when I was young (hehehehehe) I had a buck coming right down the "happy trail" like a dream. Coming straight across the field and heading for his trail in a hedge where my stand was. Same scenarios I've seen 20 other bucks do. Except this time this guy did something slightly different. He was about to duck into the hedge so I got to full draw figuring he'd continue to walk down the path like the other ones did. (haha nope) He stopped about 25 yards in front of me and stood there behind a cluster of 6 or 7 small trees. 5 more steps and he's a dead deer. I didn't time it, but like everyone else said it seemed like 5 minutes. Probably 1.5 in reality. I did what david said and lowered my elbow against my ribs whith my hand to my cheek still. I even stuck my chin behind my release clip and tried to hold some of the weight with my chin. I got to the point where I just about couldn't take it anymore and kinda let out a little groan/grunt thing. You know the kind of sound you make when you're lifting something heavy. For whatever reason that sound got the deer moving. He didn't bust out of there, but he didn't want to be there either. He took about 5 more steps out to my shooting lane and I raised my elbow again and took him.
I'll tell you what was amazing is the "new found" energy that you muster up when he finally presents you with a shot. Suddenly it seemed like I just drew back again. Body stopped shaking and bicep stopped burning.