[jglamine]

Check to make sure your windows are properly sealed. If it's an old apartment you might need to redo the calk or add weather stripping. A good seal makes a huge difference. Rule of thumb is if air can get through, sound can.

I also wear noise canceling headphones while working, usually without anything playing. They do wonders to muffle outside noise.

When I listen to music while working it's usually brain.fm - music specially designed to help you focus. Anacdotally I've found it useful, and they also have a whitepaper going into the science behind their approach.