After I finished my paid internship for the summer, my supervisor keeps asking me to help with small tasks here and there, but it'll end up becoming a lot of time. I don't want to burn bridges, but I also have school and other commitments and I really don't want to continue doing this. How do I say no without burning bridges? On the other hand, if I do continue with it, I definitely don't want to do it for free. How would I ask to be paid? Thanks in advance.
Saying no is a task necessary to learn for all professions. The key is to be polite but crystal clear that you are saying no. People try all the time to avoid outright saying no but then they wonder why the other person thinks they might mean yes. Remember it is kinder to say no clearly right awy than let someonthink it might really be yes. So what to say. In this case I would simply say: "That's sound's like it would be interesting but I cannot help out due to my workload at school."
Don't get into a long involved explanation of why you don't have time. If you don't want to, you don't need to give an explanation at all. Don't say "I can't help at this time", because then he may want to know when would be a better time.
If you do want to help but want to be paid. Then @WesleyLong's comment below should work just fine:
Joe, I'm glad to hear the project is still evolving. I'd love to keep participating. I'm done with my internship requirements, but I could sure use some part-time / contract work to help pay for my schoolbooks. Do you want to talk about it over coffee this Thursday?
If he agrees to meet, decide before the meeting how much per hour you would want to be paid and how many hours per week you are willing to work. Make sure this is clear and in writing before you do any work.
First, decide if you want to continue doing these tasks. If you can't afford the time, asking for compensation won't solve your problem.
Nobody works for free (unless you're an intern or a volunteer). Even most interns are compensated for their time/effort in one way or another. Ask your supervisor how he thinks about your effort and whether it is worthy of compensation. If it is, work out the details. If it isn't, you should stop doing it if there's nothing in it for you.
NOTE: Having a good time can be seen as compensation. You'll have to decide for yourself if you consider it as such.
Then just as casually mention that:
you have school commitments;
you can't afford to work for free;
the limited amount of time that you have available must be allocated to paying engagements as a matter of financial priority
Say that you are sorry but at this point in your life, these considerations must take priority, you wish him the best of luck and you'll be glad to help him when you get some relief from the pressure that you are under - if you so choose, you will never get relief from the pressure you are under as far as he is concerned :)
The number of ways of saying no without saying no is limited only by your imagination and creativity. And of course, you learn how to avoid burning bridges by practicing the various ways of saying no without saying no and coming up with new ones :)