State machines are handy, and along with modelling they can be implemented directly. A simple implementation is you define the possible states for your application, or module or whatever, and the valid transitions from state to state. You assign the new state when the state should transition, and this assignment is wrapped by some simple check that the transition is valid. If not, throw an error, or handle it, etc. The state path can also be logged to aid with debugging. I've found it invaluable in building web apps, finding odd things not caught in traditional testing e.g. like cases of trying to transition from the current state to the same state.