You might have heard the story of the Fisher Space Pen (also known as the Zero Gravity Pen) — legend has it that it cost millions of dollars and years to develop the a pen which works in zero gravity (and thus for the use by astronauts). The Russians meanwhile took the pragmatic approach and simply used pencils.
Great story — but mostly not true (thank god for Wikipedia — destroyer of urban myths).
There is a lesson in the story though (regardless if the story is true or not): Always, always, always start with the problem and stay with the problem.
Entrepreneurs typically start with a problem they want to solve (in the case of the Space Pen: How do we write in zero gravity?) but sometimes forget to stay with that problem throughout the process. Instead they go get lost in technical solutions, design masturbations or simply get carried away by feature creep. And end up designing something which might solve the problem but is far removed from the market (and thus doesn’t sell).
Figure out what the pencil-equivalent is to your Space Pen and you’re on the right path!