I agree that Mindmaps are the best way to structure projects.
Just put everything that comes into your head on the appropriate twig.
Either on paper or straight on the computer.

How do you transform a mindmap item into part of a next actions list?

If you're using something like Mind Manager you can copy items from one program to another.
You can also start a new mindmap for every project (everything that requires a few steps).

All this is very fine, but
a) for me it seems to be very easy to lose focus

What drill must I follow not to get distracted and keep all my projects in sight?

(Buy a bigger monitor to see more of the mindmap is no option.)