Man its tough to pick one up so i usually drag the deer to a whatever small ditch i can find, or the largest diameter fallen tree I can find. I run the front wheels down into the ditch and drive the front rack up against the dirt bank on the opposite side (which is where i've dragged the deer). Then i just roll/pull the deer onto the front rack and tie it down.
If I cant find a small ditch i pull the deer over to the largest diameter fallen log i can find and pull the deer up onto the top of the log with the legs on one side. Then i drive the atv up to the other side of the log and its much easier to pull the deer up onto the rack.
If theres no ditch or log to use I do it the old fashioned way. After field dressing roll the deer onto its back. I then stick my right arm inside the carcass up to my elbow, and use my left arm to wrap around the deers neck. Then, using a motion similar to rocking a baby in my arms i swing the deer hard and upward in an arc swing and plop him onto the front rack.
I also have just tied the handle of a regular deer cart to the back rack of the atv and pulled the deer out that way. The key to doing that is to not go too fast. Typical deer carts arent designed to move at 15 mph and the axles build up a lot of heat at that speed. I try to pull the cart out at no more than 5-6 mph.