I try and support it as close to the hinge as possible. If it has a pistol grip style stock (Tamer/Survivor/Varmint/ATI), I shoot it one handed. Well, as much as possible.
I used to try to boresight it by opening the action, sighting through the barrel and adjusting the scope. Now I just set up a giant piece of cardboard, and make some random dots on it. Then I pick a couple of dots to align the crosshairs against, and fire one shot. Adjust scope and fire another shot. Til I get close to what I am aiming at. The giant piece of cardboard allows me to pick many combinations of spots to align against. After I finally get it dialed in at 25 yards, I drag out the spotting scope and do the same thing at 100 yards. Then I try and shoot a few groups.