Depends on the bin design, but in the hot Australian summer, I removed the lid and placed some damp hessian/burlap type stuff on top. The evaporation kept it all cool, and I sat some fly screen on top to keep bugs out.

Minimize reflected light.
Wet down the area where the bin is.
I don't know if it is on cement under a roof or on grass under a tree.
Maybe don't harvest right now because the more material they can have to escape into.
Hose water is sometimes cool, so maybe after watering the special plants, water the bin. Normally I would not add water but this is not normally. I'm not sure about that. Maybe just toss in ice cubes.
Maybe we will get some thunderstorms.
Even the plantain in the shade was crunchy.
Maybe give away some worms to friends with cellars?

Removing the lid, adding moisture and keeping the air moving across the bin are all ways to cool it. Freezing tasty morsels and burying them (without disturbing the bin too much, which could cause heat by composting) is also a way that has been suggested.

My strategies for my rubbermaid bin:

- Keeping the lid off
-2 Frozen water bottles on top of the bedding
- Today's feed was a bunch of frozen chunks of melon rinds, wrapped in some brown paper, buried in the back corner of the bin. This keeps the interior cool, and also offers some moisture content.

Despite the temps, the worms seem very happy, and have been making castings quite quickly of late. I think it's all the fruit I've been feeding them.

Heatwave bonus: the fungus gnats seem to be dying off more quickly than usual. Yay!

thx for the replies. it hit triple digits here in NYC, but two freezer packs, extra moistened bedding, and a slightly opened lid seemed to do the trick. hopefully i can say the same thing for the rest of the week (summer?).