Love_XYZ
I write a chapter with average length of around 2400 words every day. I'm also a full-time graduate student and work part-time. The biggest two reasons for keeping up:
1. Proper outlining. Moving past a broad outline, which everyone should have, you should also know what you'll be writing for the next 5 or 6 chapters before you even start on a chapter. Sitting down while already knowing what you're going to write for the day will likely reduce your time spent writing and help prevent writers block.
2. Scheduling. Write every day at a consistent start time. Allow no distractions during this time and just get it done.
Other than that, practice I guess. Chapters took me five to six hours each when I started out. Now they usually only take me two to three hours.