I worked summers, while taking classes during the third year of my degree, and participated in a coop program. The summers and coop program were great, but I regret working while in classes. The money I made wasn't worth the hit to my marks and comprehension of the material. In retrospect, I'd have been better off taking a bit more debt and concentrating on academics.
Also, when my father went through school, his summer job not only paid for tuition and books, but gave him spending money and paid for his car, gas, and insurance. I couldn't afford a car until well after graduating, and even then with insurance and gas it cost a quarter of my paycheque. Had I done the same thing, I'd have needed a year-round part-time job just to cover the car.
And that's if you can get a $12/hour job in the current economy. Working halftime certainly can make a little dent in your debt, but it also makes it hard to take a full load of hard classes.
PS: His senior year he got a 13$ per hour internship at the coast guard, but he kept the cooking job until he graduated because the internship was not full time.