How do I prepare a plastic mold to easily release beeswax? A friend of mine keeps bees, and asked for help cleaning the beeswax he harvested this year. I figured out a solar oven and filtering setup which melts and cleans the wax. However, I've now got a big chunk of (clean) wax solidified in a plastic bowl, and I'm struggling to get it out!

This was a great trial run, but I'm planning to re-melt this to pour into some ice cube trays, in order to have smaller (easier to use) blocks. How should I prepare the plastic mold so the blocks will release easily? I'll hopefully be using the same technique when we eventually make candles!
Sounds like you're looking for a mold release agent.
Some examples of release agents are:

Pam Cooking Spray
Olive Oil applied with a spray pump
Industrial Mold Release Agents such as McLube

All of the above mold release agents are applied in the same manner. Starting with a clean mold, spray on a thin layer of the mold release agent into the mold, covering all the surfaces of the mold. Let it sit for a few minutes before adding your compound to the mold.
Careful not to spray too much mold release as the excess can mix with your casting compound and affect the surface finish of the cast part. A light application is all that is needed.