As you get more advanced with your workouts a good goal to shoot for when doing machine exercises is to...

"Beat The Machine!"

Now what I mean by this is to stick to a particular weight stack machine exercise as a regular part of your workouts. Each week strive to better what you did the previous week by increasing the weight by 5 or 10 pound increments.

Eventually with time you'll be able to work up to the point where you can rep out with the entire weight stack of the machine. This is what I refer to as "Beating The Machine".