Use electrolytes now that it's warmer (electrolyte tablets in your water, or use a sports drink like Lucozade Sport).
Dynamic stretches before your run (not static stretches through).
Static stretches - but away from the run (i.e. run in the morning and stretch in the afternoon or vice-versa).
Make sure you fuel properly - get enough carbs in your diet.
Also, are your trainers in good condition? Are they the right ones for you (i.e. did you have a gait analysis before buying them?)?
Sorry
@gaz but Epsom salts don't have any evidence in support of them (except a falsified study by a relative of one of the big manufacturers!).