One big monitor and one smaller one (but not small!). Tests with a third big monitor came to the conclusion that a third monitor is very useful when you do something in parallel to developing (like upgrading my Gentoo). On that note: I really like my Apple Cinematic for working with Photoshop and images as it keeps the colors "true" for bad viewing angles.
Stereo + steady music (for ex. electronica; look for 1h+ mixes on SoundCloud)
Depending on what I plan to do either cold beer (creativity++) or some cups of coffee (productivity++)
Big desk
(Physical) notepad and pen
Good keyboard
Nicely tuned OS settings (dark skin, balanced mouse sensitivity, no annoying pop-ups,...)
Good lighting (not too bright, but not too dark, either)