For intermediate storage of the brush, say several hours or overnight, an easy way to keep the brush fresh is to wrap the bristles and ferrule in a plastic wrap like a Saran Wrap. I don't like foil because it is not airtight and does not make total contact with the bristles. Plastic wrap is cheap, easy to use and gets you a nice airtight seal.
Another technique, although a bit more work, is to get a glass jar or paint bucket and fill it with the appropriate solvent for the paint you are using. This would be water for a latex paint and paint thinner for oil based paint. Take a 1 foot length of duct tape or even masking or blue painter's tape. Then place the brush against the jar or bucket and in the solvent so the bristles are fully immersed. Tape the brush to the jar by placing one end of the tape on the outside of the jar, around the brush handle and ending again on the outside of the jar.