Hang them bristles-down or wrap them in plastic wrap (for short-term storage). Avoid resting them on their bristles.

Soak in hot soapy water, then scrape off with a plastic scraper. For stubborn paint, use rubbing alcohol or acetone (test first).

No—always dedicate brushes to one type of product (paint, stain, or varnish) to avoid contamination and poor results.