To achieve a very crispy skin takes a few days of planning ahead, at least the way I do it. First in a large stockpot, I put honey, cinnamon stick, fresh ginger, and garlic. You can use any spices really but you do need honey, cider or some other sweetener. I then fill with water. While this is heating up, I take a fork and prick the skin of the duck all over, paying special attention to the areas where the thigh meets the rest of the body. Don't prick so hard as to pierce the flesh. Next I dunk the duck in the boiling liquid just until blisters appear on the skin. Remove it, pat it dry and place in the fridge for 2-3 days uncovered, preferably close to the fan. When ready to roast make sure the duck is sitting above the bottom of your roasting pan and roast slow and low. Giving it a final blast of heat just before the duck is done.
This may seem very involved but it will give a very crispy skin almost like the skin off of a peking duck. If this is too involved just prick all over and roast as stated above. It should give you a relatively crispy skin though not as crispy.  | 
