Pierre Fermat: Keep a pad of paper handy.
To Jesus Christ: Write it all down, and save copies with at least three trusted disciples*. Believe me, when people write it up 4o years from now, they’ll get half of it wrong, or twist it in ways that you didn’t mean.
- Yes, include Judas Iscariot and Mary Magdalene in that. Diversity is good.
To Martin Luther King:
“The view from the balcony ain’t all that hot.”
For JFK:
“Dude, your shoe’s untied.”
Pierre & Marie Curie: Lead-lined aprons would be a good idea when you mess with that stuff.
Henry Ford: Your son Edsel is a heckuva competent guy; you should pay more attention to him. And fire Harry Bennett.
And while we are on the Tudors: Jane Grey, you are a nice girl, but you need to run away from home. Do it. Elope and get yourself knocked up by some nice minor landowner and show absolutely no ambition other than writing a few lines to your cousins on their coronations. Your parents are using you, everyone is using you, and your head is going to end up on the floor. Mary has more power and popularity than you think.
Also, MLK, please check the ashtrays and lampshades for hidden microphones before you get your freak on.
I can’t top that.
In reverse chronological order, which means some of the later in time ones might not be necessary.
To Richard III: Execute Margaret Beaufort and John Morton, and marry Lord Stanley off to some Sweet Young Thing.
To Edward IV: For crying out loud, marry Eleanor Butler and forget Dame Grey. She’s trouble on the hoof!
To Harold Godwinson: Get some stirrups, rest your men after the battle with whats-his-face the Viking, and don’t let your men charge downhill at the Normans when they’re tired!
To (the mythical) Eve: Are you sure you want to eat that fruit?
To Douglas Adams:
Go get your heart checked out. I mean it. Right now.
To Oppenheimer and the creators of the Manhattan Project:
It’s really, really not worth it, guys, please trust me on this.
And, finally, to the crew aboard the Mary Celeste:
What was that all about?
To the supply officer for the USS Akron: Yeah, I know, rigid hull airships do want to minimize unnecesary weight. But deciding that floatation devices are obviated because you operate over land so much is a very, very bad idea. Even if you have to sacrifice one of the aircraft - get some life preservers and life rafts aboard. Adm. Moffet will be very grateful, for one.
Speaking of rigid hull airships, to anyone and everyone involved with the R-101: It’s a boondoggle! It’s too heavy. Don’t use the experimental, tempramental, and heavier engines! And for the love of kittens and flowers, get rid of that [del]stupid, fucking useless[/del] piano!
To the crews and subordinate pilots of Flight 19 - your flight instructor is an idiot. Ignore him and listen to the enlisted ATCs. Turn west and find Florida, don’t keep trying to go north!
To Knut Fraenkel and Nils Strindberg: Andree is either a weak-willed simpleton, or a charlatan of the first water. Either way, his Eagle balloon expedition is doomed. Don’t go along.
To CINCPAC, August 1945: Yanno, you’ve not heard a word from the Indianapolis in a day or so. Shouldn’t you look into that?
To George H. W. Bush: everyone’s getting vasectomies at age 22 now. Be hip! Be cool! 
To John Travolta and Tom Cruise:
Pay no attention to that Hubbard guy. He’s a fruitcake. Oh, and Tom, quit jumping on the furniture.
Or…“Put the little S.O.B.s in Military School. All of em. It can’t hurt em one bit.”
To just about anyone living before the first part of the 20th century:
“Bathe.”
To Jim Henson:
“That flu seems nastier than that doctor who gave you aspirin told you. Why don’t you go to a hospital and get a second opinion?”
Or about 700 years earlier:
"No, cats really aren’t the agents of the devil. They kill rats, and that could be a good thing.
You break it you buy it, mama didn’t raise no fool…ok, seniors mama at least.
Stonewall Jackson: You know, I will send someone to check out the lines. You should go to be early tonight.
Lee: Dude, listen to Longstreet.
Washington: Get rid of Gen. Lee. He’s no good.
Yes. It was.
To Aristotle: Emphasize repeatability and experimental controls. Tell people to not trust anything just because it’s written down, even if it was written by you.
To Galen: Emphasize hand-washing and check out some of these molds I brought back with me.
To Steve Jobs, circa 1988: Don’t piss off Bill Gates. Microsoft might not look like much now, but your pretty black boxes need software more than you need a gigantic ego. (Ah, who am I kidding? Getting Jobs to part with even a fraction of his ego is like spooning matter out of a neutron star.)
To Gary Kildall: IBM is ripe for a deal. Make one before some pear-shaped geek from Redmond does.
To Carlos, Príncipe de Viana. Don’t trust your father. Yes, I know he’s your father. But he’s also a traitorous son of a bitch.
To his sister Blanca: have you considered asking a widow or two for help? I mean, your husband evidently has problems getting it up, but maybe there’s something you guys can do other than follow his proposal of using one of his knights as a stallion. Which I agree you shouldn’t, by the way.
To Carlos V. Yeah yeah being emperor looks cute. Now forget about that shit, anyway they speak foreign up there, and concentrate on America. Yes, I know you were born in Flandes, tuff luck, not everybody can be from Spain. Well, OK, you can play around Italy too but only because we’ve already owned half of it for quite some time and they speak understandable.
Pontius Pilate : “Rule #1 when dealing with religious leaders : Don’t martyr them. And if you insist on killing them, make it look like an accident or an embarrassing death. NOT a public spectacle.”
To Washington or any random Founding Father : “Free any slaves you have, and outlaw it in the new nation. You’ll burnish your legacy, and if that means some of the colonies don’t join, that’s better than a major civil war down the line. Putting the problem off for a few generations is moral cowardice, and will NOT make things better.”
Emperor Constantine : “Don’t trust those Christians ! Give them a chance, and they’ll depose and execute you, then install a council of bishops. The civil war after that - well, this area is called the Roman Waste, and Rome the City of Bones in the future.”
George Lucas : “George. Lose the Ewoks. In fact, forget anything cutesy, and don’t wait too long to make a sequal trilogy or you’ll lose your edge.”
“Eve, girl - make sure you eat fruit from both trees. And share with the snake. Adam ? He’s a jerk; ask Lilith. Let him get his own fruit.”