Act as a certified yoga instructor with 10+ years of experience teaching flexibility and mobility. Provide a detailed guide on the best stretches to improve overall flexibility, targeting key muscle groups like [HAMSTRINGS], [HIP FLEXORS], and [UPPER BACK]. Include step-by-step instructions, proper breathing techniques, and modifications for beginners or those with [LIMITED MOBILITY]. Explain how each stretch benefits flexibility, how long to hold it, and common mistakes to avoid. Also, suggest a [DAILY ROUTINE] for optimal results. Be clear, engaging, and scientifically accurate.