How do you map user journeys for your products?

Starting a project from scratch, I often find myself scribbling on paper or in books to get my user journey mapped out. As flows get more complicated, it can be a bit of a nightmare.

How do you design and map user journeys through your product?

Do you use any techniques?
Any online tools?
If you're a solo founder, how do you remember it all?
If you have a co-founder; how do you share the knowledge / agree the journey?