Gosh your sewing machine would be so much nicer if the belt wasn't such a pain.
You know what I'm talking about if you've got one of those yellow cog belts. Sure they might not slip (as much) but they can make such an irritating scrunching sound as you sew.
Maybe you've got a machine that has a one of those hard rubber belts that doesn't squeak but it sure does slip. You could have so much more punching power if the belt would just grip. Adjusting to make the belt tighter can put strain on your motor and it still might slip.
You might try to fix this problem by using one of those universal stretchy rubber belts and soon realized that they break easily and right in the middle of a project!
The inexpensive solution that could bring joy back to your sewing experience is belt dressing! This is manufactured for the belts in automobiles. There are many different brands. The one I choose promised to, "Eliminate squeaks, clatters, and slipping. Protects and extends belt life." Could it get any better then that?
A few words of caution. This product is designed to be sprayed on large belts on your car engine. It comes out of the can fast and furious, you will make a mess if you're not prepared. After a few messy attempts trying to spray directly on my belt I now spray a small amount in a container and apply with a cotton swab. It also smells strongly, ventilate properly. It goes on wet and dries. I give my belts a liberal dose. I have not had to reapply this very often so one can will last a LONG time.
I found mine at Walmart for around 3$, it is also available anywhere auto stuff is sold. I haven't used different brands but I assume they all preform in similar ways.