First dip the brush in the solvent suggested by the manufacturer. This is done before painting to prevent paint dry-up on the bristles and for smooth application.
In the case of an oil based I would know that solvent/thinner can be used this way. What about Latex? Is water the solvent in this case? During my last exterior job I did not try dipping the brush in water because I was not sure that this would be right and I did not want to find out on the front of the house.