This topic is absolutely, unarguably, the key to exercising long into your life without breaking down your joints, suffering overuse injuries by the dozen, and keep enjoying it!
Here is true wisdom:*

If you don't stretch regularly, you will stop exercising. That's the truth*

*This comes from any number of great books on training, muscle physiology, and decades of running and coaching runners, especially aging competitors who have never stretched and see no reason to start - even if they're turning 40.

It may happen via a process that takes a few years, or due to an event in which a tendon  snaps, a muscle tears, or another excruciatingly painful disaster occurs. The technique to avoid all of this agony and frustration only requires 20-30 minutes a day, is painless, and can be done anywhere. Yet, the vast majority of amateur athletes never do it, or do it so minimally that they shouldn't bother. What is the magic technique (it's the heading!)?  STRETCHING!!!

    As we age, two things happen to our muscles, both of them bad. They get weaker and they get less flexible. Fortunately, both of these age-related processes can be reversed with minimal effort. Another, more pragmatic reason to stretch regularly is that nearly all running injuries are caused, either directly or indirectly, by muscle tightness and a lack of flexibility. The muscles that are usually to blame for injuries are the hamstrings (four muscles, of which the biceps femoris is the strongest) and the calf muscles (made up of the gastrocnemius and soleus muscles). The calves, in particular, are responsible for lots of misery, which is strange considering that stretching them is one of the more comfortable muscle stretches you'll encounter.

Un-stretched, shortened muscles take on several unhealthy characteristics, as illustrated in the drawing to the right (be sure to click to get the larger image); these include pain and stiffness, muscle spasms, longer recovery time due to reduced blood flow, development of painful trigger points, and a much greater tendency for tendonitis due to the increased friction as the muscle sheaths rub past one another.

Calf Muscles

    The calves are comprised of two muscles - the gastrocnemius and soleus. As the left diagram shows, the soleus is flatter and wider than the gastrocnemius; it is also considerably stronger. There are two stretches to do here, one for each muscle. They look similar, but the feeling each gives is easy to differentiate; these simple drawings are quite accurate in showing the right form for both the gastrocnemius and soleus. It won't take long before you can easily feel when the gastroc or soleus is being stretched. If you're not positive that you're stretching each muscle individually, consult with a PT or other pro.

If you don't do these stretches carefully and regularly, the most common resultant injury is plantar fasciitis. When the calf muscles are tight, when you plant your heel to start a step, you pronate (roll to the inside) to relieve the stretching muscles. This, in turn, stretches the plantar fascia - the tough band of tissue connecting your toes to your heel bones, to keep your arch from collapsing. It then starts to rub against the calcaneus (heel bone) and hurt a whole lot (docs call this "exquisite pain"). It hurts much more in the morning or after long siting, when there's been little blood flow to the foot.

If you're showing the symptoms of plantar fasciitis, see a podiatrist immediately!! - preferably one who's familiar with running injuries. I did this, and didn't have to take a single day off from running. ;-) If you don't, and foolishly try to run through this injury, the most likely result is that you'll tear your Achilles tendon from its insertion point on your calcaneus (heel bone) which not only hurts like hell, but will take at least a year to recover, including surgery and endless hours in a pool. Sound like fun? Not to me, either.

Quadriceps Muscles

Another essential muscle group to stretch is your quadriceps, which is the group of four large muscles on the front of your upper leg. The illustration to the left shows all of the important legs muscles, especially the four quads - the sartorius and the three vastus muscles intermedius, lateralis, and medialis (please click the image to see its fill size).

A fortunate aspect to stretching your quads is that it actually feels pretty good! The most common technique is shown to the right, although it needs refining: your upper legs should be about parallel, not with the knee pulled back like hers. Your spine should also be straight, so that you are isolating the muscles you want to concentrate on. You always want to be able to feel the muscle being stretched!

Hamstring Muscles

Stretching the hamstrings, unfortunately, is much more uncomfortable and difficult (the folks in the right and left photos are each doing a different hamstring stretch), but it's essential. Before I started stretching religiously, the longest leg of a race I ever did was the final 1/2 mile of a local 5K. My hamstrings were already a little sore at the start (BAD OMEN!), and became progressively more painful as the race progressed. By the 2.5 mile point, I simply could not run any more, and limped in the final 880 yards, in incredible pain, knowing what I had to do to return to racing. It was absolutely the pits.

    There are several ways to stretch the hamstrings. One problem is that they're incredibly strong, so it can be hard to pull them with sufficient force to get an appropriate and effective stretch. The first photo in this section shows about the easiest way. Just put your foot up on something that's about pelvis-high, straighten your other leg and point its toes toward your elevated foot, and lean into it until it gets uncomfortable. Hold this for 20-30 seconds, ease out of it, and repeat it twice more.

    NEVER stretch your hamstrings by trying to touch your toes while keeping your knees straight, and ESPECIALLY DON'T BOUNCE!! I don't care what they told us in gym class in the 1960's - that technique is murder on your lower back, and bouncing sets up a reflexive contraction in the very muscles you're trying to stretch, causing them to shorten, so it's actually counterproductive! The guy in the left photo, on the track, is using a technique that works well only for those with already fairly flexible hamstrings. The real show offs, I mean, limber folks, can touch their chins to their knees! Disgusting. ;-)

General Procedure for Stretching any Muscle Group

As with most physical exercise techniques, there are usually a few ways to stretch muscles correctly, and a multitude of incorrect procedures that will, at best, do you no harm. Unfortunately, it's very easy to become injured, sometimes seriously, when doing a stretching routine. The good news is that avoiding injuries is quite easy, just by remembering a few simple rules.

First: NEVER STRETCH A COLD MUSCLE GROUP! Why not? What happens if you place a rubber band in the freezer, take it out, and stretch it? It snaps! Your muscles won't snap, but you can do real damage. I recommend (and practice) running about a half mile (or a few minutes), then stop and go through the stretches. You'll find that you can stretch further and with much less discomfort by doing it this way.

Second, AS WITH ALL STRETCHES, stretch the muscle until it is SOMEWHAT UNCOMFORTABLE, BUT NOT PAINFUL. If you cause pain, the muscle will reflexively try to contract to keep from becoming injured. Hold the stretch for AT LEAST 20 sec., then slowly ease out of it. DON'T EVEN THINK ABOUT BOUNCING!! Stretching should be slow, with no sudden motions. After you do a different stretch or two, repeat this one. You should do at least three reps per session.

Final Thoughts

It is impossible to over-state the importance of maintaining a functional stretching program, with which you continue to gain flexibility as you gain in physical conditioning. Think of your stretching as part of your runs - that you'd never do a run without stretching.

As I said above, never stretch a cold muscle. You also don't want to leave stretching until the run is over, because you'll be tired out and are unlikely to give your stretching the attention it needs. My own practice is to run for 1/2 to 1 mile, slowly, then stop and go through your stretching routine. When you resume running, you'll be amazed at how good and loose you feel!

How long should you stretch? The rule of thumb is that you should spend 10-20% of the time you spent running on your stretching. So, if you're going for a 30 min. run, and it takes you 40 min., you should stretch for between four and eight minutes. After awhile, when you've been stretching as part of your run for a time, you probably won't bother timing your stretching. You'll also probably find yourself stretching for longer than the suggested minimum, as you feel more and more how much better a runner you are when your limber and stretched-out!

Good luck, have fun, and don't even think of skipping this part, or your running career will come to an abrupt, painful, frustrating end, long before its time.