There are a number of techniques to employ to reduce the risks inherent in taking nitrous. A lot of them are common sense
, but people that are high often lose any sense at all
First, remember that nitrous is not oxygen. Oxygen keeps you alive; nitrous doesn't. So don't fill a garbage bag with pure nitrous and tape it over your head. Don't open a tank wide up in an enclosed space. And if you want to reduce suffocation risks altogether, hyperventilate first. This also allows you to hold it in longer, which will get you higher.
Second, remember your Pressure/Volume/Temperature ratio from physics class. Remember that as pressure decreases and volume increases, temperature drops rapidly. Put the near-freezing gas in an intermediate container (like a balloon) before inhaling it. Don't use a plastic bag, use a balloon! It's much, much harder to suffocate yourself with an object that doesn't fit over your head.
Third, remember that you basically lose consciousness when on nitrous. When you pass out, be somewhere that this won't be detrimental, like a couch or a chair or laying on the floor. Balconies and tightropes are a bad idea. Standing might not be so bright, either.
Finally, like any substance (even food, air, and water), overconsumption has side effects. With nitrous, extended high dosages results in the depletion of vitamin b12. Over time, this depletion can result in damage to bone marrow, along with a host of other maladies. The few cases where people wound up in the hospital due to b12 depletion involved using dozens of bulbs per day for months at a time.
Follow these guidelines, and don't do anything stupid, and you won't get hurt.
As a side note to Ælien: I'm not going to tell you that your cousin didn't die, though making up such a lie seems to be a standard last resort of anyone deathly opposed to drugs. I will tell you that there must be more to the story than simply "he took a hit and suffocated." Either he had a bag over his head, or he had a medical condition that caused his heart to stop. Otherwise, when he passed out, he would have resumed breathing. You can't die from holding your breath.