Prior to dehiscence no pollen will transfer to the paint brush bristles. The best time for collection is when the the pollen is a rich yellow color (for immediate pollination). For storage, it be preferable to be slightly drier. If the pollen has dried to the point of becoming light in color, it will transfer to the brush with difficulty and may actually drop off the brush, rather than adhere.
The stamens can be removed from the flowers, or the entire flower can be removed, if they're ripe and close to releasing the pollen. By placing them on a tray under a gentle desk lamp the pollen can be harvested rather quickly, but this shouldn't be done prematurely, as the anthers may simply shrivel and fail to release the pollen. A paint brush may be unnecessary when using this method, tapping the anthers with a tweezers may work better. For storage, the pollen should not be moist, as in using fresh pollen.