A lot of good points have already been discussed.

Emphasis was put on silence as the key. I respectfully disagree.
Humans cannot comprehend how well a deer's sense of smell actually is.

I put more focus on trying to minimize the deer's abilty to smell me than anything else.
Once I enter his woods, I try to eliminate as much as I can, his ability to realize I am in his back yard. Keeping movement to a minimum, keeping as quiet as possible - they all help - but keeping my scent away from him is paramount. Sometimes, I believe he can smell my color orange.
The deer can hear you, see you, smell you or simply know you're there.

Shooting a deer is very simple. Simply put the sights on his vitals and shoot him. Not rocket science to shoot a deer. Now, putting yourself in the position to put the sights on his vitals and shooting him is the difficult part. That's where scouting, practicing, understanding the animal we intend to hunt - all come into play. This may sound kind of trivial but remember, you can't shoot them if you can't see them. Follow you instincts after scouting and if you may have setup in a wrong location, it's OK to reloacte. I remind myself that deer have to be somewhere all the time. It's up to me to determine exactly where in the woods that is.

A couple other quick points.
If I plan to hunt from a stand, either on the ground or 20 feet up, I always play over in my head exactly what I will do if I suddenly see or hear a deer behind me, over my shoulder, a placeI did not anticipate the deer to come. That way, I will be a little more prepared. One of the most popular comments made by hunters is: "How did that deer get so close to me without me knowing it".

Expect to see and shoot a deer every single time you enter the deer woods. Be positive. Plus, expect to see a deer at any time, at any place. Especially those places that we, as hunters, know deer should not be.
If you move around a bend on the road, over come to the top of a ridge, always be prepared to see deer. When you let your guard down and you suddenly realize there he is, you're busted. You may play that scene over in your mind for years to come. I always remember that for me to be successful, I have to see him before he sees me. Most of the time, that's a lot easier said than done.

Finally, have fun. That's what it's all about.

Good Luck to all this upcoming season!!.....Jimmy