It takes major time management skills. I don't get any aid/loans at this point so I'm paying for everything on my own. As a result I work 32-40 hours per week at one job, have a flexible research asst second job (read a book, abstract it, send it in), and take 8 credits in the summer and 18 in the regular semesters. (The full-time job is a new addition so I'm still figuring out how it will really work.) This summer I'll be working an 8 hour shift Friday, Saturday, and Sunday, plus one or two other shifts during the week. I'll most likely be sleeping in two 4 hour shifts because of the way scheduling works out, and probably won't really be studying on days that I have both work and class. The fall will be better because I'll have all morning classes so I'll work graveyard and go straight to class and then come home and crash.
I have no social life, but in the end it will be worth it. The computer and tv will be turned off for most of the next year, but as long as you keep to a schedule (which you absolutely must write out and stick to) it's definitely doable.