Natural light from the window instantly brightens up the small bathroom. If possible, do not decorate windows with clear or frosted glass.
Make the most of your windowsills, as the light gives the effect of an open space. Remove all clutter and install sheer curtains or blinds in light or pastel colors to maximize your chances of getting sunlight throughout the day.
Consider installing skylights in loft conversions. These are great for letting in as much sunlight as possible without worrying about privacy issues.
For bathrooms without windows. Choose lighting fixtures that bring as much electric light into the room as possible. Do not go for heavy or domineering bright shades.
If possible, add lighting fixtures around the room, for example, sconces on either side of the mirror. If you have the space, hang a large mirror to reflect the light around the room and paint the walls fresh with a bright or fresh white color.