Ask HN: How do you keep your thoughts together when building a product?

I've tried building products in the past and what ends up happening is that the ideas and features keep getting compounded and I end up in a place where I no longer know what I'm building.

When building a product thoughts and ideas are coming in from all over the place. How do you keep your product feature set from getting unwieldy, and how do you stay focused on building what you initially defined.