In reply to Ross C: ross describes the best way, generally called finger clipping. By pulling down slightly with the finger you steady the krab and prevent it twisting which can be a problem with pinch clipping. However qds should be placed to set the gate in a direction that minimises the liklehood of the rope unclipping itself in a fall not to always face a certain direction to clip, the petzl catalogue/website used to have diagrams that explained this better than I can.
Finger clipping can be used whichever way the gate faces. In ross' example when clipping with the right hand, if the gate faces the right then you finger clip with the thumb instead of your finger. With practice finger clipping soon becomes automatic and you will be able to clip qds facing either way with either hand. It is an important skill as otherwise when you are tired you can wate time fumbling clips while hanging on one hand and of course mid clip is when you face the greatest fall potential.
The real question is whether if you pull too hard or for too long with your finger/thumb does this count as cheating?