is they are water based them warm water and some mild soap should do most of it.
you can wipe off excess paint on a tissue or napkin or something. and change your water every once in a while while painting.
for non water based use thinner or brush cleaner. then wipe off the excess on a tisue, and then clean with warm water and soap.
if you have paint where the brush meets the handle that happens often. if your brush can take it you may want to try a little rubbing alcohol on it. just use q-tip or cotton ball with alcohol on it and wipe handle->brush with it the alcohol will loosen the paint so that it comes off.
then you can was with warm soapy water to clean off the left over bits.
never put your brush into fingernail polish remove from my experience as the acetone can eat the glue holding the bristles onto the handle.
hope this helps.