Selection first, of course, then I usually score the edges, and work my knife in gently to peel - like skinning a deer. Birch and cedar tend to come off in sheets more readily then some others. Your choice depends on intended use. Juniper comes off and shears off into into tinder-size bundles when you work it in your hands.
Knife selection is key, too. A thick blade can be harder to slide behind the bark without tearing it. Thin works better.