Prune your pepper plant when it's about 4 weeks old, remove the top flowers (called the crown flowers) and select the two strongest top branches, pinching off the weak ones. This will make a Y shape, and double your harvest. You shouldn't need to prune much after that, but you need about 10 leaves per fruit on the plant to maintain enough photosynthesis, so you can pinch off any excess to try to promote reproductive growth. Here's a good diagram.