Like any other activity that operates machinery, don't do it while you're under the influence. You need to be honest with yourself about what that means. I don't know if there is a hangover like there sometimes is with alcohol, but definitely do not smoke pot and fly an airplane while still under the influence. If there is any reason to raise a suspicion, like if someone tells an official that you smell like weed or are acting like you're under the influence of drugs, then they are going to shake you down and possibly request a drug test.
If you have an accident and there is a suspicion that you are a marijuana user and were indeed under the influence (like say someone knows you were on Reddit asking about the consequences of smoking pot and flying) they could have grounds for testing, suspension and/or revocation.
As an example, there was a pilot who left the taxiway and hit some airport signage. No damage to the plane but some damage to the signage. Because he was commercial operating for hire he was immediately drug test and license was revoked. Apparently the pilots story was that he smoked a joint at a bachelor party the weekend before, the test doesn't really lie. Now I can't specifically elaborate if it was the company or government who tested him but the end result was the same, license gone, he'll never fly again.
Also, I know of an automobile driver who was pulled over and while the police searched his car they found the remains of a joint in his ash tray. The remains were old but it was easily enough to drug test him and charge that he was "under the influence". With marijuana staying in your system over a month (and from what I understand, drinking a bunch of vinegar/apple-juice/cranberry wouldn't of helped him) He was technically under the influence weeks after he smoked pot.
Now, I am not under the impression that every time an aircraft crash happens, there is a mandatory drug test but look at Harrison Ford, I am most positive that he had blood work at the hospital and in that blood work they could pull a drug analysis, just like if you were DUI car crash (depending on the state).
Lets say you break a reg by flying through an altitude or causing a runway incident. The controlled that day is especially bored and feels like doing paper work. Unfortunately he uses you as his opportunity when you failed to follow his instruction. You receive the "phone number" and are alerted that the FAA will be contacting you about the situation. Without reason to believe you screwed up because you were high, I do not think you would have a drug test coming your way.
On the other hand, lets say you taxi in to another aircraft with people on board and while the event is unfolding someone mentions that they think you're acting funny, you would probably be test for being under the influence (of something).
It is also important to keep in mind two things. First, marijuana legalization is a very different conversation today then it was 5 years ago. Because it is legal in some states (but superseded as illegal by the fed), you could make an argument that you went to Colorado and smoke a recreational joint a few weeks before. Your argument would then be to prove you were "under the influence", just like if it was alcohol related and you blew a .03 BAC. It's not illegal to drink and fly (more than 8 hours later), it's illegal to fly while being impaired under the influence of alcohol.
The other way I can look at it is this: I fly for a living and travel all over the US/CAN/MEX. I fly 5 days a week to 1 time a month, depending on schedule. I don't want someone dangerous operating in the same airspace as me. I don't fly with a hang over and I'd appreciate the same professional courtesy from others. I don't touch booze a full 24 hours before a flight. Some safety issues I go above and beyond because it's not worth taking a chance. Other situations I follow to the letter of the law. It's bad enough when thousands of low time, low flight recency, weekend warriors all launch seemingly simultaneously as soon as the IFR conditions pass but now there's Cheech and Chong doing touch and go's while stumbling all over his radio calls?! NO THANK YOU!
TL:DR Be a responsible pilot/human being and refrain from doing anything that is going to impair you (and endanger others) while operating heavy/life threatening machinery. A lot of flying is common sense stuff. Don't be a dick.