Be organized. Keep a calendar of your school assignments and your work schedule, and revisit it as often as possible. Be disciplined and fight the urge to procrastinate.
Get your friends and family on board. Recruit them as your cheerleaders and to encourage you if they notice you're slacking off.
Budget your time well. Figure out ways to create more pockets of time in your schedule to attend to your schoolwork. Wake up earlier a few days each week, spend half of your lunch break doing research for class, or get some reading done on the bus or train on the way to work in the morning, for instance.
Reward yourself. Don't get burned out by your rigorous schedule. Find time to relax.
Look for ways to integrate work projects with school projects and vice versa.
Register for night classes or courses that meet only one or two days a week. Alternatively, change your work schedule to accommodate your school schedule, if possible.
Re-evaluate things after each semester. Determine whether attending school full time is right for you. You might want to consider a certificate-based program or professional-development classes instead.