Partying - Other
Since my poison of choice is alcohol, this page is not as complete as if I had first hand knowledge and contains mostly just common knowledge stuff. If you have a fondness for mind altering substances you will definitely want to research this more.
I'm sure you have seen stories about Burning Man being a drug fueled sex orgy in the media. Forget that. Sure there are drugs there - just like there would be in any city of over 60,000 residents, but it is not as prevalent as the media would have you believe.
Nevada has some of the harshest drug laws in the country. The various LEOs (law enforcement officers) will be happy to bust you if they catch you with drugs. There will be undercover officers there. Getting busted will really fuck up your day. Some years enforcement is more zealous than others and it ranges from oppressive to unobtrusive.
Undercovers - Sometimes these seem pretty obvious, but I suspect that is just a smokescreen to throw people off. To quote Eric: "There is a reason they are called "undercover", and you can bet that anyone who's doing undercover out there has been going longer than you, has better costumes than you, and has access to a better art-car than you could ever hope to be on. If they were "painfully obvious" then all the undercovers watching gangs and criminals would be dead. Think about it."
The Stupid Tax
The fine for pot possession is commonly called the stupid tax since usually it is the result of doing something stupid - like sparking up out in the open. The LEOs have night vision gear - so even darkness is not a guarantee of not being observed.
Speaking of sparking up - the smell of weed could be used as probable cause and get your camp tossed so consider other methods. Your camp mates may not be very happy if your behavior causes the entire camp to get searched.
A general rule you should observe is never do more than one stupid thing at a time.
Although they are legal to buy for use in making whipped cream, inhaling them or paraphernalia for inhaling them is illegal so beware.
Generally accepted wisdom is to not give to or accept illegal substances from someone you do not know outside of the event. If someone approaches you asking for some - the chances are very high that they are an undercover officer. Burners tend to know better than to do that.
The Federal Government does not allow medicinal use and Burning Man is held on Federal land - end of story.
Even if you can at home that won't help on the playa, and in fact I've heard that having a medicinal use card has been used as probable cause for a search. I'm not sure how true that is, but I wouldn't doubt it.
Much more could be said on this subject, but I'll leave that to folks with more first hand knowledge.