It's true: The
directions left and
right are really hard to
define in terms of other
spatial relationships.
Other directions aren't so tough. Take up and down, for instance. "Up" is away from the ground, out into space, the opposite of the direction gravity pulls in. "Down" is towards the Earth, away from space, and it's the direction things go in when they fall.
Or, consider forwards and backwards. "Forwards" is the direction your face points, the direction it's easiest to walk in, and the most comfortable direction to look in. "Backwards" is the direction your butt points in and when you walk a ways, the place you were earlier is in that direction.
What can we say about right and left? Well, right is the opposite of left, and left is the opposite of right. It's a wonder any of us remember which is which.