I'd say the pomodoro technique is only reserved for studying and focused work but would not carry you through the entire day. I have tried it but didn't like it personally though. FYI I'm not an entrepreneur though :/
Energy and motivation ebs and flows. Priorities shift multiple times a day. The environment is never constant. There's no single system that works in my view. And trying to make every hour productive is also a recipe for burnout - unless you call rest and tending to yourself when deserved or required also productive.
I tend to constantly change my approach.
To manage my time - sometimes I'm calendar strict, sometimes I run a quick to do list, sometimes I use my intuition (We usually instinctively know what's important). I find being calendar strict (Everything goes on calendar - EVERYTHING) highly effective when cramming a lot of work, but not sustainable long term and unforgiving in my line of work where random "disaster" and unpredictable effort/failure/success looms around every corner (IT).
To manage my motivation and discipline - sometimes I create a game of it, sometimes I use my hobbies as fuel/incentive, sometimes I just take leave when I must.
There's no solid answer I'm afraid - else there wouldn't be a hundred books/techniques on the subject.
