The first thing you must try to do as a defender is get in the right frame of mind. Concentrate on getting every hit up. Nothing can get by you. You're an animal. If you know that you are the best player out there, you'll probably play like it. Then again, you may get your butt thrashed, so play smart too. Anyway, keep thinking about digging the ball.
Know your blocker
More importantly, trust your blocker. Your partner is going up on the attack to stop the hitter from swinging at his area. He may also try fancy stuff that fakes the hitter out, but no matter what, you must have faith that your blocker will cover his designated area. I have been guilty of not trusting my blocker because I was sure that the hitter was swinging for that area. I get burned everytime and look like a moron at the same time. It is also likley that your blocker will get a touch and deflect it to your area. For example, if he's covering line and you scoot over because you see the hit is going line, your blocker could deflect it hard angle and you're too far out of position to have a chance at the dig. This has also happened to me a lot.
Know your opponent
Know how he thinks. What are his best shots? What are his weakest shots? Where does he often hit? What does he do with a bad set? Does he dink a lot? These are key questions to understanding where to be when a hitter is swinging. If you can know what the hitter is going to do, you're one step up.
Hide behind your blocker
This is something Mike Dodd says to do. As the ball is set, position yourself so that the hitter cannot see you. This should make him a little confused, since he won't know what side you're covering. Once the set is up and the attacker's eyes are on the ball, move over to your area. I have sometimes used this tip, but it seems to work only on the best hitters. Since better hitters look at where their hitting and not-so-great hitters (like me) just go up and try to hit the ball in bounds. I have also added a little trick of just faking out the hitter. This means that as the ball is set I may be positioned right in the middle of the court and as he goes up I move to the side I'm covering. This almost always works. Just give the hitter a wide open area, then move to that area. It's great!
Take away the good and easy shots
This means letting the hitter have the hit that paints the line, or even the incredible cut that barely makes it over the net. Chances are that the hitter will not make these shots all the time. If you can take away his good shots that are easy to dig, you force him to adjust and make the more difficult shot. And you didn't even have to do any work!
Get into position
This means getting into a proper passing stance. Feet should be staggered, one slightly in front of the other. Your arms should not be together. Keep your arms in a position that is comfortable and makes it easy to the low digs and the high ones. I usually keep my arms pointed down since I can get my hands up fast to get any high hits. Also, stay on the balls of your feet and lean slightly forward. This is because it's a lot easier to dive forward than it is to lean back. For this reason you should be about 8' from the back line. That way any hit that is above your shoulder is probably out and you're ready for any short dinks. This position stuff may actually be a little useless since the hitter will probably not hit the ball right to you (unless you're really good). Essentially, get comfortable, keep moving around, and be ready to explode to any point on the court to get the ball.
Call the ball
This usually only applies to dinks over the blocker. Sometimes the blocker will try to turn and hit it up, but if you're in a better position let him know.
Get it up
Get the hit up by any means possible. This could mean diving all-out or reaching high up with one hand. It doesn't matter how pretty the dig is, so long as the ball is still in play. Also, get the ball high enough to give your blocker a chance to get to it, since digs don't always go where you want them.
Prepare to hit
As a defensive specialist, you will probably receive most serves and spikes. This means you must be prepared to hit the ball. Once you dig the ball get into hitting position. A dig doesn't count for any points, but a kill does. The crowd will "oh" and "ah" at a great dig but fall silent if you can't do anything with it.
Many people say that you should react and not anticipate a shot. For outdoor volleyball, those people are wrong. I have found that good defense is about 25% reaction and 75% anticpation. Normally you can watch the set and see the hitter's most likely shot. In this case be prepared for it, but be ready for a trick. A lot of people say you should line up with the shoulders of a hitter. I have found this works only against bad hitters. A good hitter can trick a defender by not taking his "shoulder-shot". The only good side to this hit is that most hitters can't swing quite as hard when they do this. The best way to judge a hitter's target is to look at the set. If the set if way outside and high be ready for an angle shot. As soon as I realize what shot the hitter is taking, I get to his target. Sometimes I'm wrong and I get burned, but usually I'm in the right position for the dig. Just remember, don't guess a hitter's target, take what you can see from the set and what you know of the hitter and adjust your position accordingly. Remember to keep moving, you must defend your court (usually about 2/3 of the full court, if you're blocker is doing his job). You must be able to reach any point on the court to get a dig; this means that you must keep moving.
I hope these tips help. If anyone out there
thinks I missed something or gave stupid advice let me know through email.
Thanks for reading, and happy digging!