Always know where your towel is. A towel is just about the most massively useful thing an interstellar hitchhiker can carry. Partly because it has great practical value. More importantly, a towel has immense psychological value. Any man who can hitch the length and breadth of the Galaxy, rough it, slum it, struggle against terrible odds, win through and still knows where his towel is, is clearly a man to be reckoned with.