This game is frighteningly addictive. The problem with it is that to get a score worth anything you have to play for hours and hours, sometimes days. The highest level you can get to is 49, at which point there are 50 balls on the screen.

The game is deceptively simple, and at first the strategy that you use will get you no further than level 10. This consists of catching balls in large traps and shrinking the traps around the balls. The problem with this is that when you get more than 9 or 10 balls on the screen it becomes impossible to build the initial traps.

A more refined method is to click once near the corner right along the side so the wall going down (or right, left, up or whatever) hits the wall then you move out just enough to allow two lines between the wall and the edge and start another wall. This will build a trap without you having to sacrifice a ball. This picture might make it make more sense.

  |  Two black lines thick between the clicks.
  |   | *2
  |   |
  | {--} 
Click on 1 first, wait for the wall going down to form, then click 2.

Then you wait for a ball to come down your trap and once it hits the bottom click directly in the middle. It'll grab the ball without taking up any extra space. Then you just wait for the next one to come down. This allows you to get killer bonuses because you don't waste any space. With practice this becomes second nature.

Note: This becomes impossible at extremely high levels, where you must use much more difficult means to avoid sacrificing a ball. I haven't mastered this yet, but I've gotten it to work a few times. The kind of trap you form is the same and once you form it you continue to collect balls in the same way. It's just a different way of forming it. What you have to do is click two black lines away from the edge and at a very small height (assuming vertical traps) because of the number of balls moving around. But the tricky part is that you have to click at the exact moment that a ball is directly above (or below) your cursor. This stops the wall going up (or down). Again, this picture should help.

   |  Two black lines thick.
   |  |
   |  |
   |  | o
   |  | *

The hard part of course, is timing it so the ball blocks the wall moving up. This takes huge amounts of practice.