If you've cooked sous vide as much as I have, you've probably run into this scenario: you set up your sous vide cooker for some smoky pulled pork—a recipe that requires a full 24 hours cook—turn the water bath on, and then leave it unattended while you go about your business. You come home from work the next day to find that the water in the container has evaporated so much that the sous vide cooker has shut itself off for who knows how long. (For what it's worth, so long as the bath hasn't been off for too long, you can usually just top it up and keep cooking.)
Why You Should Add Ping-Pong Balls to Your Sous Vide Bath
How to Optimize Your Sous Vide Technique
Not only can cooking sous vide in an open pot lead to situations like this, it's also a waste of energy. To combat this, I do two things. The first is to cook in a plastic container, like a cooler or one of these eight-quart Cambro storage containers. Plastic is a much better insulator than metal, so you're saving some money on your power bill right off the bat.
What about the evaporation? You can wrap the top of the container with plastic wrap or aluminum foil to prevent the escape of excess heat and water, but a better way to do it—short of cutting a device-shaped-hole in the lid—is to use Ping-Pong balls.
By floating a solid layer of Ping-Pong balls on the water's surface, you'll simultaneously insulate your bath and help steam to condense and drip back down. The great thing is that ping pong balls conform to the shape of whatever container you're using, you can easily drop and lift bags from the bath mid-cook, and they're completely reusable. I keep about 50 of them stored with the rest of my sous vide kit.
UPDATE: A couple people have questioned whether it's actually easier to store ping pong balls than to simply use plastic wrap or cut a hole in the lid of the container. There are a couple other compelling reasons to use ping pong balls. First, plastic wrap or holes can lead to long term problems as they end up venting any escaping steam directly towards the head of the sous vide unit, causing condensation near the electronics (not all sous vide devices have sealed electronics). With ping pong balls, any escaping steam comes out evenly from the top surface.
As for drying the balls after use and storing them in between uses, I've found that the balls don't really require drying. Drain them, put them back into the empty Cambro container, and let it sit for about 10 minutes and excess moisture evaporates pretty quickly because the water was already hot from cooking. I store the balls directly in the container, so when I'm ready to cook, all I have to do is add water. The ping pong balls are already there and ready to go. That's faster than using plastic wrap!
Of course, everyone's habits and kitchen set ups are different, so this may not be the end-all solution for each of you.