Clean paintbrushes are happy paintbrushes.
Sure, you can use the “wrap-in-foil-and-stick-in-the-fridge” trick if you get in a jam, but the real key to getting a paintbrush to last a long time is to clean it. And it really doesn’t take that much longer than rummaging around for foil.
Run the brush under the faucet and flare the bristles out so the water sloshing around in the bottom of the sink can get up inside the ferrules (the metal part). After the water runs clear, use a brush comb to comb the brush.