You should have fixed ideas about what you want to achieve with the time you have, and don't spread yourself too thin. Ask yourself what your biggest priorities are.
Give yourself reasonable targets and deadlines, so that you don't waste time planning unrealistic ones. Don't just invest yourself in work. It's important to have a healthy social life too, for your personal growth and development.
The key: Make lists of things you have to to in order of their importance: priority, important, unimportant.