Let me start out by saying that this is a big responsibility for either party. Choose your spotter wisely and make sure that you spot to your abilities.
- Pay attention to the person you are spotting at all times. Don't look at the pretty girl across the room doing deep knee bends.
- Be prepared to help the person being spotted instantly.
- Ask the person being spotted if they would like you to put your hands on the bar. (Some people do and some people don't)
- Ask the person being spotted how many reps they think then can handle on their own.
- Make sure that you can handle the weight that you are spotting. If you don't think you can handle it don't be macho and help anyway.
- Provide only the effort that the person requires from you. Don't jerk the weight around while someone is trying to lift it.
- Offer encouragement to the person lifting the weight. This really helps the person lifting the weight. They might be able to squeeze out those last reps.