...from a former nationally ranked distance runner (OK, a zillion years ago, but still.... ;-).

The number one thing in becoming a better runner is to keep running (duh). Especially as one gets older, this means avoiding injury. To avoid injury:
1) Never skimp on the shoes. This doesn't mean that you have to buy the most expensive pair (because sometimes they are most expensive because they are the newest model), but when you find a pair that fits you well and has loads of cushioning, don't settle for less than that. Consider also buy a pair of heel cups for additional cushioning.
2) Don't over train. Give yourself a day off once or twice a week. Don't run mega miles every day. Give yourself some "short days".
3) Watch your feet for a while as you run on a flat surface. See if you pronate or suppinate (tip your feet inward or outward, thus running on the edges of your feet instead of down the middle of your sole). If you do, consider seeing an orthopedist to see if orthotics might help correct your stride, thus making you less injury prone).
4) Have an experienced friend or coach watch you run. Better yet, videotape yourself. See if you have form-flaws that are correctable (e.g. an asymetric stride, immobile or over-still arms, posture problems etc.). Correcting these can help prevent injury and also can shave substantial time off of your "times". The more efficient your stride, the less energy you burn/step, the faster or farther you go for the same physical condition.
5) If you are finding yourself suffering from minor injuries then something is wrong. Correct it because it will only get worse. If you get shin splits, for example, you either need new shoes, you are overtraining, or you are overweight. I STRONGLY recommend that if you are substantially overweight, you avoid running altogether. Find a low-impact exercise, like swimming, until you get your weight down, THEN take up running. It's not that running is not great for losing weight, it's just that the damage you do to your joints may be irreversible.
6) Stretch!!!!! Before and after. But warm up with some slow jogging before you stretch at the beginning of your workout. You don't want to injure yourself stretching. Never bounce a stretch. Always just hold it gently for a few seconds and then release.

Now apart from avoiding injury, some of these tips depend on whether you just want to see your workout times come down, or whether you want to be the best competitive runner you can be. For the former; just run (baby). Run long and slow. But keep within yourself - if you overdo it, you might not only get injured, you might get tired of doing it and give it up. So just run as much as you enjoy, and do it regularly.

For competitive runners, some additional tips:

1)) Light weight training. Mostly upper body and core. The running will take care of your legs. Most competitive runners would tell you that its never their legs that get tired, it's their arms or their gut.
2) Interval training. Do it 2 or 3 times a week. Mix it up. Make it a little harder than you think you can do. Mix up the intervals. Sometimes don't vary the distance (e.g. run the same distance over and over - for example, run 1000 meters 5 times with 1 min rest between each repeat). Sometimes vary the distance (e.g. do a step up/step down - 100, 200, 500, 1000, 500, 200 100 meters, with short rest in between). You might need some specific advice here. Designing a good interval workout can be tricky and depends on how good your conditioning is. But in general, you want to run the intervals fast enough to get winded, and the rests should be short enough that you don't fully recover your wind before you run the next interval.
3) Hill training. Even if you are not going to run hills competitively, it gets you in great shape.
4) Same thing with high altitude training.
5) If you are going to run in a competitive environment that includes hills, then LEARN TO RUN DOWNHILL. Most runners don't know how to run downhill and end up expending huge amounts of energy slowing themselves down. You know, you lean back a little and with each foot plant, you try to control your downhill speed. Why? If your goal is to go fast, let the hill do some of the work. To do this, imagine yourself leaning forward to make your body perpendicular to the slope of the hill. I don't mean bend at the waist. I mean imagine yourself keeping good posture, with your body straight upright, but bending forward at your ankles to place your straight/upright body perpendicular to the slope of the hill. You won't, in fact be able to get yourself perpendicular. It is more of a mental thing to keep your foot strike from exerting force backwards to slow you down. Try to imagine your foot already moving backwards when it strikes the ground.
6) Relax when you run. Don't keep your arms stiff. Let them move, hang them down as much as possible. Keep your jaw loose. If you are not relaxed when you run, then your antagonist muscle groups are working against each other and you are wasting energy.



If you are having minor nagging injuries, like shin splints,