During the Run
Most people don't realize that running properly reduces injuries. When running, think smooth, think light, and think efficient. Because of those three things, your body shouldn't have any jarring impacts that would lead to you injuring yourself. When you run properly, the force you're exerting helps you go forward and should you run on a technical trail (rocky and dangerous terrain), you would be able to catch yourself and what would normally end up as a sprain wouldn't be anything at all.
Run at Your OWN Pace, Your OWN Distance
Running at a pace that is far beyond what you could do and a distance you can't do is one sure way of getting injured. Remember the 10% rule. You can only increase your weekly mileage by 10% so that your body can be accustomed to it. I would also suggest that you have a slow-down week about once a month to allow enough rest for your legs.
Of course, when you run, your eyes ought to be open, but for real, keep your eyes on what's in front of you, a nail sticking up from the ground, a loose rock, a tree root. Anything can cause you to trip up can end up injuring you, so the more things you eliminate, the less you'll have to worry about.
During your run, your body can sometimes tell you that it's tired... when it does, you can continue on. However, if your body is telling you that it's gone too much, you can change your long run into a shorter run because you're doing what's best for your body.
Run at Different Speeds
Acclimating yourself to different speeds makes your body have to adapt and therefore become stronger. The stronger the body is, the more it can handle. Running at different speeds allows you to work on both speed and endurance.
Outside of Running
Stretches and Warm-ups
Keep yourself limber because tight muscles are more likely to tear. I'm not too much of a fan of stretching, but one important part of stretching before you go run is warming up. When you do warm-ups, it allows your muscles to loosen up, which means that you'll be less likely to tear it.
Posture isn't all about what you do while you're running. It has to do with what you're doing outside of running. Keep your posture and you won't hurt yourself.
Listen to your body, and take a break if you need it. There's such thing as over-training, and when you do that, you force your body beyond what it can handle... leading to injury. Stop when your body says to.
Running works on certain muscles while cross training helps the other muscle groups get stronger and also helps with the overall balance of the body, allowing the same muscles to get different kinds of workouts. Helping the overall fitness of the body prevents injuries.
Rest and Food
Without resting, your body wouldn't be able to recover, so you need the rest... and you also need the proper nutrients.
Before you get to the point where your muscles and ligaments hurt, you can feel it coming. At that point, make sure to wrap it nice and snug to insure that it won't go get to the point where it's injured.
Put the Right Stuff On
Blistering and chafing occurs when there's friction between you and something else (or yourself). In order to prevent that on the run, tighten your shoes, put lubricants on to lessen the amount of friction, and wear material that doesn't rub against the skin.