Good morning my friend. Here are two exercises for looser hamstrings.
Just doing these 2-3 times a week (perhaps at the end of your gym workout, as I did today) should increase your mobility and flexibility.
Plus you get some protection against injury while running or lifting.
Of course, simply keeping your body limber, supple, and active throughout the day is more important than a dedicated program of stretching!
Have a beautiful day!