Act as a certified personal trainer with 10 years of experience specializing in flexibility and mobility. Provide a detailed guide on the best exercises to improve hamstring flexibility for [BEGINNER/INTERMEDIATE/ADVANCED] individuals. Include step-by-step instructions, proper form cues, and common mistakes to avoid. Tailor the recommendations for someone with [TIGHT HAMSTRINGS/INJURY HISTORY/NO PRIOR FLEXIBILITY TRAINING]. Also, suggest a weekly routine with [3/4/5] sessions and the ideal duration for each stretch. Explain how these exercises benefit overall mobility and prevent injuries.