I call it "my groove" or "plugged-in", and it's not something I can force. I notice it generally involves two things, the first being the standard from most replies, which is music, and the second is that I am working on a problem that I enjoy working on or if I'm not enjoying it I see the problem as a challenge. If I am failing at making progress though, it will quickly fall through, it's the momentum of being awesome that compounds. The same is true in the games I play to let off steam.