Lol zebrah on "lots and lots of yoga"
You're going to want to look at anti-inflammatory herbs. There are many. One of the number one reasons for inflexibility is inflammation of the muscles caused by daily wear and tear.
One of the most common concoctions is 2 tablespoons of apple cidar vinegar (from the mother is best) in water (I prefer apple juice) and 2 tablespoons of raw honey. If you use apple juice, eat the honey before you drink it.
This is great for arthritis and inflamed joints and muscles as well as overall great for your health. After drinking 1 glass everyday for a week, you'll notice the difference.
2 tablespoons of raw honey alone, everyday will also be a great asset if you just can't stomach the acv. You can mix it in tea, smear it on toast instead of jam, or just eat it. Who doesn't like honey?
Of course as Zebrah pointed out, stretching is essential. And epsom salts soaks in a bath will also help your muscles to relax.
Be sure to brush your teeth after drinking. Vinegar and acidic food in general can wear down the enamel on your teeth if left to sit there.