I've always said that the commute is everything between your desk and your house, so when I have to wait extra time on the elevator for people who still don't understand how it works, that counts.
You're on the elevator. Someone else is approaching. Maybe you want to hold the door for the person (or maybe you don't), maybe the person thinks she can make it. Either way, at least a few times a week I see the panic moment "Oh crap, I've forgotten how to keep the door from closing!" Whether it's staring blankly at the button panel trying to figure out which one is the "door open" button, or hurling yourself directly into the path of the door and letting it slam on you, neither are particularly good options. If you're going to get on I'd prefer you just do it as efficiently as possible, especially if it's morning and we're going to repeat this little scene half a dozen times as people pile on.
Here's the trick. Most (if not all?) modern elevators don't rely on the "open if you bump into something" approach on the doors. Instead they have a little light sensor that detects whether there is something blocking it. Sometimes it is at eye level (like my current building), but I used to work in a place where it was down closer to your knees.
Find that little sensor. Now, when the door is closing, regardless of which side of it you are on, just put your hand directly on that spot. Don't panic about it, don't freeze and crack under the pressure. If there's still an inch or two of clearance where you can fit your hand (or even a folder or piece of paper), you're fine. Block sensor, door opens. If I am the first to walk on an elevator and there are people behind me, I'll always put my hand on that spot, assuring that the door does not begin to close on anybody.
And for god's sake, if you do want to get on, stop standing directly in front of the stupid thing when the doors open. People may want to actually get off, too.