I was going to type a more detailed response, but the people below have done an excellent job already. The body has to be balanced, distractions at a minimum, the mind open and curious and the problem "not too easy and not too hard". I like to think of it as "chasing the white rabbit". One thing that can help is to write a detailed plan of what your coding session is going to achieve, with some stretch goals. If you do it right you will enter a cycle of achieving small goals leading to refactoring and bigger goals and then you.... look up and the whole team has gone to get Tacos without you again.