Never reload, because it's not fair to the bullets that haven't been fired yet.
Don't actually shoot your target, because that get's blood everywhere, and blood is icky. Just aim carefully, and kill the air around them.
Don't run, because that wears out the bottom of your boots, and combat boots are expensive.
Always keep your knife facing down when you move, because if you trip you don't want to stab yourself in the face.
Make sure to bring a baseball bat for use as a grenade launcher, it's much cheaper. Just try not to miss the grenades.
It's always polite to high five the blades on a helicopter when you get on or off of it, otherwise you'll hurt it's feelings.