The most important rule while prototyping is to be efficient. I love Pareto's principle which says around 80% of the consequences result from 20% of causes or efforts. In game development terms, it means that 20% of your development effort will result in 80% of the playable outcome of your game. Think about it, when developing a feature, you spend a certain amount of time getting the feature up and running, but most of the time, you catch the edge cases and/or polish the feature.