It can be tricky to find the balance between working your full-time job and pumping life and energy into your freelance or side work.
First of all, there's the fact that you only have so many hours to work with, and so much brain power to contribute. Everyone has to rest! And not to mention, you have to watch out for issues like conflicts of interest, non-compete contracts, and colliding deadlines. For example, if you created a special way of setting up a JavaScript menu at your full-time job, you might not be allowed to use that same menu with your private clients. Or if you have a major deadline at your full-time job AND in your freelance work, you'll have to choose to prioritize one or the other.
I've found that the keys to freelancing with a full-time job are transparency, passion, time management, drive, and, perhaps most importantly, coffee.