The biggest step is to be sure the bathroom has adequate ventilation. An exhaust fan is important. If you choose to install an exhaust fan in a bathroom be sure it fents outside the structure. (If it does not vent outside then it will create mould in the area where it is vented.) Other actions you can take: keep the shower/tub clean – removing soil and soap film (which contains skin cells removes the organic material mould needs for food; leave the shower curtain or door half-open (to increase ventilation); do not store towels or other materials in the bathroom since they can be food for mould.