Large Language Models (LLMs) exhibit sensitivity to prompt variations, making systematic testing and improvement essential to ensure accurate, relevant, and cost-effective outputs. Regular testing minimizes unnecessary API costs and potential misinformation, and the article details a step-by-step approach to prompt experimentation and evaluation using tools like Helicone, which allows for real-world data testing and comprehensive logging. Effective prompt testing involves logging requests, creating and evaluating prompt variations, deploying the best-performing prompts, and monitoring them in production, with evaluation metrics tailored to specific goals such as faithfulness or coherence. Helicone stands out by enabling testing with actual production data, offering an intuitive interface for prompt management, and supporting A/B testing and side-by-side comparisons. The article emphasizes that prompt engineering should be a data-driven, iterative discipline, leveraging both human evaluation and automated LLM-as-a-judge methods, with the ultimate aim of enhancing user experience and resource efficiency.