Flash units, just like cameras, should be kept in cool, dry places when not in use. If you're not planning on using your flash for a few weeks or more, take the batteries out to avoid damage from battery leakage. This doesn't happen very often, but when it does it can cause serious damage to delicate equipment.

If for some reason you won't be using your flash for a longer period of time, keep the capacitor working properly by firing it up once a month and discharging the flash a few times.