The biggest mistake most people make when starting ANY project - business or otherwise - is not having a CLEAR PLAN.
Excitement is usually the driving force behind a startup. This is important BUT it must be strongly tied to a CLEAR PLAN.
And this is the KEY to remember when it comes to a plan.
The #1 purpose of a plan is so that you (and all key people involved) have a CRISTAL CLEAR IMAGE in mind about what you are creating.
If you cannot 'see' something - then you will struggle to create it. And the clearer you can see something, the easier you will be able to create it.
The #2 purpose of a plan is to use it to share your ideas with others so that they can SEE what you are wanting to create.