The easiest thing that comes to mind is close your bedroom door. That way kitty can't get in, and if kitty can't get in she can't pee on your bed.

Otherwise, if there is some reason you can't or don't want to close your door, try covering your bed with plastic (garbage bags, etc) or tin foil. Kitties supposedly don't like the feel of these materials on their feet and will therefore avoid the area.

Good luck!