Play the body not the puck
When an offensive player has the puck look at the player's chest, not the puck. If you stare at the puck the odds are that you'll get beat. While starring at the players chest you can still see the puck. Poke check but still play the body.
Tie up offensive players
One of the keys to playing defense in ice hockey is to tie up offensive players near the net. Keep your stick under their stick and lift it just as they're about to get the puck.
Keep shooting lanes clear
In addition, keep the shooting lanes clear for goalie to see the puck by positioning your body between their's and the goalie's viewpoint.
Don't just look at the puck
Don't get hypnotized by the puck and keep track of who the most dangerous player is to score. Keep looking around for open players in front of the net. Someone behind the net or in the corners is much less likely to score than some one in front.
Teamwork with your other defense man
Always know where your other Defense and center are before going to the corner for the puck.
When to pinch?
When on offense and playing the point if the puck is on the other side of the ice, move over to the center of the ice and be prepared to back up your defensive partner. If you partner is deep then back up even more.
Don't pinch unless you know that your defensive partner is backing you up.
Don't pinch if your forwards don't back you up.
Skating backwards is a key technique for defenseman. Practice, practice and more practice. Don't be afraid to fall when learning. If you don't fall you won't learn.
Point your toes together and you should start moving backwards.
Force oncoming players to the outside
When an attacking player has the puck, force them to the outside (towards the boards). If you're on the right line up your right shoulder to their right shoulder and if on the left, line up your left shoulder to theirs.
Dealing with really fast skaters
If there is a really fast skater approaching you then don't skate backwards facing them, skate forward with your back to them. Give them the outside and force them towards the boards.
Clearing the puck
Practice clearing the puck high off the glass as to not ice it. Also practice flipping the puck out of the zone.
Learn from your mistakes
Whenever you make a mistake learn from it. Find ways to incorporate what you've learned into a practice drill.
When face-offs in your zone make sure to take the man who goes to the net. Talk with your forward to make sure he knows to guard the point.
Center plays the slot
The most important defensive player is the center. Your team will lose games if the center isn't picking up the man in the slot.
Think defense before offense
Whenever you're on the ice look for threats such as:
Communicate with the other players on your team.
Lift your passes
Learn to lift your passes. A pass on the ice is much more likely to get intercepted, but if you do a saucer pass it's harder for the opponent to get the puck
Don't go down to block a shot
If you're reading this I'll assume your not a professional. Only professional's should block shots. it's too dangerous for everyone else. You can stand up and block it (don't position stick where puck could hit it and ricochet into your neck, gulp).