Its actually not that easy for some people to hold things level. Especially if there's any sort of movement involved.
Sort of related, but kind of OT. When you've been in the Navy, you can walk with a cup of steaming hot coffee, filled close to the brim, one hand and not spill a drop. Its a pretty neat trick, especially when you're walking down a p-way (narrow steel corridors - passageway) in moderate seas (boat will be rocking back and forth, or slamming up and down - forward). What is the secret? Why do people watching and balancing spill their @#$% all over the place, because you're reacting from your eyes, to your brain to your hand. You can't react fast enough.
So what do you do? Your body has a natural "gyroscope" in it, the equilibrium. Take your eyes off the cup, hold it in front of you, arm cocked comfortably and loose enough to let your muscle control take over. You will be amazed at how well, your body compensates for all the movement. Its why you notice waiters and waitresses, never look at the tray, until they're swinging it down. They're using the same concept.
/OT