"All Rook Endings Are Drawn." -- Tarrasch
Knowing that should really improve your rook endgame play. 
Seriously though, to get better at the endgame you're going to have to bite the bullet and study endgame positions and techniques. There are many good books (Pandolfini, Seirawan, Silman), DVDs (Mueller), and blogs (mine!) out there to learn from.
And fine, here are some excellent general guidelines. Enjoy!
I have been playing chess for 7 years, and I have pin-pointed my weakness: the endgame. I find it so easy to lose that advantage I just had or drop 2 pawns. I need a list of endgame principles.