All the above is good advice. Remember that if you are planning on adding stretching prior to your run, you MUST warm up your muscles before you do so. Any sort of rhythmic limbering or repetitious large-muscle activity will do. 5 minutes at least.
Also, consider that the concept of a "tight" muscle has to come from somewhere. Muscles are not tight by osmosis or alignment of the stars. Muscles that are "tight" are such because they are often contracted to maintain stability and integrity to the area to which they attach.
If the body perceives that a limb or joint needs stability, the motor cortex sends efferent stimuli for the muscles of this region to contract in order to increase stability to the area. If your ankle is an unstable one, you might be suffering from an oft-contracted gastrocnemius, in which case, when placed under strain of running, could lock up altogether. It would do this resulting from the "stretch reflex," where a muscle's nerves sense that is being stretched beyond a typically tolerable range. If the muscle has become accustomed to always being contracted, the eccentricity, or release of that contraction is going to feel like an over-stretch, and the muscle will contract deeply in response to the reflex in the Golgi tendon organs of the muscle's tendons. Enter your muscle cramp.
I know that's a lot of science, but hopefully it made sense.
How do you solve it? Not necessarily just by stretching. Obviously this is going to help, but remember that if the muscle is programmed not to stretch, stretching it isn't necessarily going to solve the problem. The muscle is contracted in response to a perceived weakness. So if you can strengthen the area the calves control, as well as the calves themselves, stability will be increased, and muscle tension will decrease. I recommend ankle strengthening exercises, calf raises, and GENTLE stretching.