People generally have a hard time when they first start using the Leap Motion due to some misconceptions most people have. Don't worry, you will be using it like a master in no time!
1. Stay at least 4 inches away from the device, it detects in a cone. It will not register anything very close up, and the closer you get to it vertically the closer you have to be horizontally as well for the device to recognize you.
2. Anything it detects must have line of sight to the device. If you place your right hand above your left it cannot see it, or if you turn your hand so that your index finger blocks view of your other fingers it will not detect them.
3. Some gestures are easier than others. If you can't get the gesture down at first make sure you are not having problems with numbers 1 and 2. It has a better time detecting swipes and simple movements than complex gestures. It picks up the palms better than the finger tips.
4. Avoid bright lighting shining onto the device. The device works by shining infrared light onto your hand and measuring how it comes back. Almost all lighting contains infrared, and can interfere with tracking.
5. If your arm is getting tired, rest your elbow on your thigh or desk. Most apps can still be used in this position and it will help prevent monkey arm.
6. Use small, precise movements, close to the center of the field of view. The edge of detection for the device changes based on lighting conditions. When you go near the edge this often results in stuttering or stopping. This will also help with monkey arm.