Washington: Free your slaves
John Adams: Don't try to take dictatorial powers for yourself
Madison: Free your slaves
John Quincy Adams: Win re-election
Lincoln: Don't see any plays at Ford's Theater for any reason!
Grant: Don't appoint corrupt government officials
Hoover: Don't run for President
FDR: Desegregate the armed forces
Truman: Don't run for re-election in 1952, it'll just be a embarrassing disaster
LBJ: Get troops out of Vietnam ASAP, seriously avoid that area like the bubonic plague and disregard the domino theory non-sense
Al Gore: Convince Thurgood Marshall not to retire from the Supreme Court until February 1st, 1993
Obama: Get the f!cking healthcare website right so that there are no bugs! Seriously, don't f!ck this one up