For this Friday, something short and thoughtful: The principles on which I was trained as a historian by my legendary dissertation supervisor.
1. Never think that you are the first to do something.
2. Always start from a solid documentary foundation (And 1 document is enough).
3. Never think that you have said the last word on something.
4. Never automatically believe what anyone else has said or written: Always question authority.
5. Never think something is good just because it is new.
6. Never think something is bad just because it is old.
7. Never let the perfect be the enemy of the good.
8. The best project is a finished one.
9. At a conference, never go to dinner with a group that is planning to divide the bill evenly. (Otherwise known as Cardinal Rule of conferences #1)
10. Seldom change your mind about historical problems once you have dealt with them.
Taylor Stoermer 
I copied these down a few years ago and just came across the notes (not like I could have ever forgotten, though).
Fairly applicable to life in general.
Oh my. My advisor was Ronald Hoffman aided by Ira Berlin, John McCusker, & Lois Green Carr, but they never taught me rule #9. (Or that one primary source was ever enough.)