One of the classic books to help you think about design is "The Design of Everyday Things" by Donald Norman. He gives great real-world examples. For example, if you design a door well, you should never have to add labels that say "push" and "pull." If you want them to pull, put a handle; if you want them to push, put a flat plate. There's no way to do it wrong, and they don't even have to think about it.
This is a good goal: make things obvious. So obvious that it never occurs to the user to do the wrong thing. If there are four knobs on a stove, each one next to an eye, it's obvious that each knob controls the eye it's next to. If the knobs are in a straight line, all on the left side, you have to label them and the user has to stop and think. Bad design. Don't make them think.
Another principle: if the user does make a mistake, it should be very easy to undo. Google's image software, Picasa, is a good example. You can crop, recolor, and touch up your photos all you like, and if you ever change your mind - even a month later - you can undo your changes. Even if you explicitly save your changes, Picasa makes a backup. This frees up the user to play and explore, because you're not going to hurt anything.
