Yeh panel beating is another skill of its own. You need to sand the panel as a whole, not concentrate on getting one area smooth before moving on to another area. And this definitely needs to be done with a block, there is absolutely 0 chance of getting a flat surface if you are sanding by hand.
A trick the panel beater I work with taught me is to rub a piece of glass before you feel your part, and then close your eyes when you do. This really sensitises you to the ups and downs of your part. It also helps, once you think your surface is close, to spray a light coat of primer over the surface just so that the surface is all one colour. Varying shades of body filler etc can all mislead your eyes, so a uniform colour helps. Once you've done that, rub the primer off again using your sanding block. This will reveal any high/low spots etc.