Okay, this is what I do. (God, I can't believe I'm admitting that I'm this ghetto).
Okay, first, put the can in a tub of warm water. I read this either on Erowid.org or here. It makes more of the NO2 rise to the top. (Urban drug myth or truth, don't know, but it seems to work. Placebo effect is amazing sometimes).
Then, start to hyperventilate yourself. Just like you used to in elementary school, or before you participate in underwater-breath-holding contests. Do that for like a minute. (Random fact: you body can start to go through hyperventilation effects from taking as little as 1-2 too few breaths in a minute).
Now, blow out all of your air and grab the can and suck like a $20 hooker. Usually what will happen is like the first 2 times or so you do this, you'll get a lot of NO2, so you'll be all set. Just make sure you're sitting down and all of that disclaimer stuff.
Now, realize that you're a poor college student, or just too lazy to buy a cracker and bulb. This is a sad realization, but it is with these thoughts that you start to get inventive.
Now, put the can back in the water bath. Now, do the steps again, but before you suck in the NO2, which by now is almost non-existant from the top of the can, you need to do some work. A lot of NO2 is still in the can, just being held down by the cream. So here's what you do.
First, start tapping the sides of the can, much like you would do with a can of soda that had been shook up. This will start to make more the NO2 come out of the cream. Then (here's the special technique) shake the can (lightly) in a circular motion. Like mostly up and down with a bit of side to side so that you're making an oval shape. Again, you're just trying to get more of the NO2 out of the cream. And finally, after doing that for like 10 seconds, tap all of the sides on a coutertop or something to try and jostle a little more NO2 out. Then suck like there's no tomorrow. If you can't get enough, as always, suck in some air to put on top of it. And you're set. (And Ghetto-Fresh!)