Decision fatigue is the principle that each decision you make requires mental energy. As your day progresses, this energy source will be depleted.
This is the reason lots of people skip out on good habits in the evenings, and the reason why you should do your most important things first.
Preserve your decisions for the really important things. Routines will save valuable brain cycles for things with bigger impact. I know that this can sound a bit extreme, but this is the reason why Steve Jobs, Barack Obama and Mark Zuckerberg always wear the same clothes.
Be attentive to the decision fatigue of others. Be specific when you need something, especially if you need a decision from someone else. Offering clear options will often speed up the process.
Know when you're tired. You might not think so, but your decisions will be notably worse. Depending on what you want to achieve, make it easy to choose the right thing. Have healthy snacks at hand, that kind of stuff.