How do you keep studying when you're not in school?

I am spending my last semester in school, and I have some courses (Numerical Analysis, Knowledge Representation, Combinatorics, etc...) I wish I took before graduation but couldn't due to time constraint.

I want to be studying even when I am working as a software engineer, but I found it hard to keep motivated enough to study after work when I was working full-time as an intern. Even when the work load was moderate at where I did my internship.

I hope my study of the subject would count in case I apply to a master's degree program in Applied Math, but I cannot afford anything expensive.