One concept that is useful (so to speak) is use cases.

When I first tackle a problem, I start from a very specific, concrete level: e.g., when value X comes in from the satellite, it should be compared against value Y and if it's greater, store it to Z. I don't bother with abstraction (in this case, a state machine) at this point, I just write down everything I can think of about how the system should respond to input to produce output.

After I think I've got a handle on the concrete situation, I then start to think about what classes and patterns will best effect this behavior in the design.