Turn power to the helicopter off by powering down the remote control console. This will help prevent leaves, twigs or vines from wrapping around the rotors. Look in the trees to get a visual on the aircraft's location.
Have two friends hold a sheet for you, while you gently shake the tree (if the tree isn't overly large, as in the case of a small spruce or pine). Should the copter fall, the sheet will be there to prevent the dislodged craft from hitting the ground.
Tie a small stone to one end of some nylon rope. Throw the stone over the branch that's holding the helicopter, taking care not to hit the craft with the rock. Catch the stone on the other side of the branch; hold both ends of the rope and pull down to bend the branch downward. Be prepared to catch the unit if it falls.
Climb the tree, just to the height where you can safely touch the helicopter to push it out of the branches, allowing it to fall into a waiting sheet. Only do this if you feel comfortable climbing, the tree isn't overly tall, and the tree has several climbable branches.