If blowing away a bunch of foreign civilians is your idea of a great way to blow off steam,* do not join the US Armed Forces*. They frown on this behavior. Sign up as a military contractor instead and you can scratch that itchy trigger finger all day.
If you’re thinking of hiring a hitman, don’t. They’ll roll over on you. If you want someone dead, do it yourself. If you can’t bring yourself to do it, you don’t want the person dead badly enough.
The only way to truly get rid of a body completely is to eat it. If you can’t go that far, you don’t want the person dead badly enough 
Forget “roll over on you”, it seems from the news that everyone trying to hire a hitman accidentally gets a cop instead.
Second corollary: don’t keep a diary either. If you were already keeping a diary, burn it.
Don’t go on television, crying and begging 'anyone who knows where [Sharon/Lydia/Helen] is or what happened to them.
Don’t confess.
Typical crime drama:
“You said you were in your office at the time of the crime. But the chair there is too far from the table. So either you pushed the chair out to stand up; in which case why didn’t you push it back under the table, or you were never in that office”!
“All right, dammit, I killed the bitch! She never cared about the business, never knew what it meant to my father… (monologue continues)”
Similarly don’t tell Mr Bond the details of your Evil Plan before you off him. Just off him and smile evilly while stroking your white cat - otherwise your hired goons will all be shot, your beautiful assassin will be seduced then killed, and you will only escape with your life.
Don’t blurt out facts that only the criminal could know. It’s amazing how many characters have been brought down by this mistake.
“Never go back to the scene of the crime”
“Get rid of absolutely everything used in the commission of that crime: all of your clothes, shoes, your cell phone (if you had it on you).”
“If the victim was in your car, and there wasn’t a previous, innocuous situation in which the victim was in your car (that other people can testify to), get rid of your car.”
Make sure the body is disposed of far from the scene of the crime.
Do not talk about Crime Club.
Leave the gun.
Take the cannoli.
Quail hunting can work in a pinch.
If you rob a convenience store at 4 in the morning after the first snow of the season, don’t drive down two blocks, turn, drive four more blocks, then park and count up the take.
This happened in Warren, Ohio, and there were three directions that they could have gone that would have put them on a highway or major artery within three blocks. Therefore the cops didn’t check out the tire tracks until after they’d taken the report.
They must have been bad at math. Even with two of them, they were still counting when the cops came up to the car.
Just make sure all your victims are human. Hurt a dog and you’re in trouble.
Better yet: Don’t go 68 in a 55 zone.
Even before I read this, my contribution to the thread was going to be Drive carefully and obey all traffic laws on the way home from the crime.
In short I’d suggest Premeditation.
iirc this advice came from Mr. Double Tap, a hitman associated with Kirksey Nix:
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kill your victim outside
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kill from a distance without identifying yourself to the victim
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never touch the victim [when you approach for the coup de grâce don’t riffle around for his wallet or wristwatch]
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walk from the scene to your exit transport which is out of visual range of the scene. if possible your exit vehicle should be in close range to an interstate highway or major thoughfare. this vehicle shouldn’t be linked directly to you.
If you invade a country in a way that turns out to have violated international law, have the UN re-write their laws after the fact.