In my university, they always emphasize and hype about UML design and stuff, in which I feel it is not going to work well with game structure design. Now, I just want a professional advice on how should I begin my game designing? The story is I have some skill in programming and have done many minor game such as getting some 2D platformer working to some extend. The problems that I find about my program is the poor quality design. After coding for a while, things start to break down due to poor planning (When I add new feature, it tends to make me have to recode the whole program). However, to plan everything out without a single design flaw is a bit too ideal. Therefore, any advice to how should I plan my game? How should I put it into visible pictures, so that me and my friends are able to overview the designs?

I planned to start coding a game with my friend. This is going to be my first teamwork, so any professional advices would be a pleasure. Is there any other alternatives than UML?

Another question is how does "prototyping" normally looks like?