Act as a startup growth strategist with 10+ years of experience in optimizing user acquisition and retention for early-stage companies. Craft a detailed guide on how startups can effectively implement A/B testing to improve [specific metric, e.g., conversion rates, user engagement, or retention]. Include step-by-step instructions on how to [identify key variables to test], [set up and run experiments using tools like Google Optimize or Optimizely], and [analyze results to make data-driven decisions]. Provide examples of successful A/B tests in startups across [your industry, e.g., SaaS, e-commerce, or mobile apps] and highlight common pitfalls to avoid, such as testing too many variables at once or ignoring statistical significance. Conclude with actionable tips for scaling A/B testing efforts as the startup grows.