This article is from the Miniatures Painting FAQ, by Brenda Klein email@example.com with numerous contributions by others.
It depends on your paint type, mostly. For acrylics which are
water-based, a good careful washing with warm water and dish
detergent is fine. Remember to re-form the tips into points
before storage. For oil-based paints, your best bet by far is
to buy a bottle of thinner made by the same company as your paints.
Not all paint is formulated the same and thinner is often
Also, Badger brand "Air-Opaque ready-to-use- cleaner" for
airbrushes does a wonderful job of getting dried paint off of
paintbrush bristles, either acrylic or oil-based. It costs $4 for
16oz. (Thanks to Bill Gilliland for that tip.)
While we're at it, there are three `nevers' to brush-handling.
Never let your brush rest in the water or thinner on its tip.
That's the surest way possible to lose a fine point.
Never scrub a good brush across either miniature or blotter.
Never let paint dry on your brush. This'll fray the bristles
into an unusable mass.
When cleaning a brush while painting, gently rotate it against
the side of the solvent/water container until the bristles stop
exuding paint. A gentle wipe across the blotter before washing
the paint out of the bristles both saves solvent/water from
clouding prematurely and helps get rid of traces of paint you
can't readily see. A clear solvent/water container is desirable
so you can monitor its cloudiness and how clean the brush is