I have run for many years and I have naturally poor flexibility. I have started yoga and this has improved my flexibility. If you can’t afford regular yoga classes buy a few basic yoga DVD’s and practice at home all you need is a yoga mat, which are very cheap.
After you have warm up for about 5 minutes it is very important to stretch properly this may help prevent injuries. Make sure the stretch you preform are static stretches not ballistic stretches. Ballistic stretches are the stretches that you see some professional athletes perform where they stretch