While the Clamp is the most widely used move and the one that you have the best overall control of the ball and most options with, there are some counters to the Clamp that work very well.  One is:

The Jump (aka Jam, Punch, Top)
The counter to a clamp:

1) Contrary to popular belief, you do not want to travel too far over the ball.  There should be very little clearance and room from your stick/glove and the top of the ball.

2) Low center-of-gravity is key here, as you want to bury your helmet over the midline and into your opponent to stand him up and make him unathletic.  It will also prevent him from seeing the ball and reacting to a tie-up of hands if there is one.

3) Punch your left hand forward just as you would on a Clamp, as this will push the opponent off of his Clamp, and also move your hands forward and to the right as you would in a Drive Clamp (Clamp Down the Line).

4) If you travel over the ball cleanly, use your shaft or fist of your glove (while still on the stick) to rake the ball back between your legs, or if you move your hands far to the right on the jump your left glove hand will be near the ball and you can use your fingers to flip it through your legs or out to a wing.  Be inconspicuous with this as it is illegal and is a lazy way to lose a draw if an official sees you. 

5) If you end up in a tie-up of hands and sticks, keep driving your helmet and body into your opponent to gain leverage, and this is also a great time to grab your opponents stick and even rake the ball between your legs with his stick if he started to get a good clamp on it.  Keep your hands on your shaft while you do this.  Should not get seen by officials if you got your body over the ball properly.

6) The key is to get as close to and over the ball as you can after the whistle.  This will give you the strength, leverage, and athleticism over your opponent to control the draw.

The Jump is a classic move, yet can often be beat by the Rake.  One way to avoid being beaten this way is to convert the Jump into a Rake of its own.  It is a faster means of obstructing the Clamp then the conventional Jump as well. 

The Jump Rake (Copyright: Andy Towers)
The NEW counter to a clamp, but for a fast-break:

1) On the whistle, punch your left hand out and up, as if you were trying to punch the right ear of your opponent.  You should simultaneously roll your left wrist back towards you during the punch as if you were accelerating on a motorcycle.

2) You should hold the stick especially loose in the right hand, letting the shaft pivot between your right thumb and right forefinger during the left hand punch/wrist roll, similar to the front hand when shooting pool with a cue.

3) The left sidewall of your head should now be obstructing the Clamp of your opponent.  The scoop of your head should also be somewhat preventing a Rake.

4) With your right hand, use the open face of your stick head to scrape or rake the ball to the side or to a 45 degree angle forward for an easy scoop.  Do not forget to hook-step right and seal off your opponent for the loose ball. 

5) You can add some efficiency to the Clamp prevention of this Jump Rake move by extending and keeping stiff the remaining four fingers of your right hand while you grip the shaft with the thumb right after the whistle during the left hand punch.   Your fingers should be jamming into your opponents stick head and disabling his move.

The Jump Rake

(click to play video)