What I do first is grasp the knob and apply light and continuous turning pressure on both directions. In the next step, I use a small hammer and hit the face of the plug, making sure not to hit the knob or lever and risking damaging it. You will be surprised at how quickly this opens one of these locks.
After several times of doing that and once you develop a feel for the procedure, you can open those type of locks in a matter of seconds. The reason this procedure works is that the locking mechanism is spring loaded, and by hitting the face of the plug, you are actually bouncing the two locking cams into the open position for a split second. That is why you need to engage the knob with light and continuous pressure to unlock it. Check out this link for more interesting locksmith tips.