Amount of work
First, you need to consider how much you can work. Decide how much time you need to go to classes and study, how much time you need to sleep and add in some extra time to relax. What's left over is what you can work.

If you can't seem to balance all that you need to do -- working enough to pay for school while still finding the time to study -- consider alternating semesters. Work for a semester to save for school, then go to school the next semester and so on. It will obviously increase the duration of your life as a student, but it will benefit you in the long run with better grades and less debt.

The hours
You'll need to make your work and class schedule fit together. If you find a job during the day, make sure the classes you need to take are available in the evening. You may need a job with flexible hours to allow you to attend class and study for large chunks of time.

Pay
Better-paying jobs usually come with sacrifice. You may need to work late hours. Or early hours. Or you may have to give up your weekends. But never sacrifice study time in favour of employment income, no matter how enticing. After all, what good is working to pay your tuition and then flunking out?

Interests
No one expects you to stay at one job all through university. After all, your university years are a great time to explore the different career options available to you. Think of all the things that interest you and see if you can find a job related to those interests.

How to find a job
The best job-referral source might be staring at you across the dinner table every night: your friends or family. Campus bulletins and career centres are often good outlets for job leads. Try a temp agency and let someone else find you a job. Scour the classifieds section of the newspapers, particularly if your school produces its own. Or you could drop in on stores you frequent to see if they're hiring (an employee discount at your favourite retailer is a nice perk). And if you're keen to find work in the field you're studying, ask your professors and faculty for ideas.