How do you keep the lids to the acrylic mediums from sticking to the jar once they've been used?
You are finding out the hard way what good glue our acrylic binder is! Any bit of paint or gel in the jar thread can be a problem, but it can be hard on an artist to have this happen.

Before you put the lid back on, take a damp paper towel and try to clean the threads out as best you can. If you discover it after the fact, try to peel the dried film out as best as you can so the lid can be tight and so little chunks of dried gel don't contaminate the paint inside.

If you have a hard time opening the jar, try our method of getting it loose. Hold the jar with the lid upward in one hand securely, and at the very edge of a table with an extended edge (like a kitchen table) whack the lid at the very edge. This should break the lid free from the jar. Sometimes you have to whack the edge a couple of times, but we normally don't break the lid or jar doing this.