There are a few cardinal rules when decorating a small bathroom:
# 1) Use VERTICAL space! Just because you don't have the room for under the sink storage or a whole cabinet doesn't mean you can't add shelving up the walls.
#2) Plan out and organize the space with the items you have. Each type of item should have its own bin or space neatly in place. Make sure you throw out old or empty bottles of hygene products and keep the clutter to an absolute minimum. And Clean often!
# 3) Paint the walls only a couple of shades darker then the ceiling (I always like a high-gloss in the bathroom or kitchen for easy cleaning and to prevent water stains). The ceiling should be a very light color, so you don't feel like its a cave.
# 4) The flooring matters: Choose a neautral color only 1-2 shades darker than the wall color. If you can, use the same tile on the floor and on the walls/shower wall so there is a sense of flow and cohesiveness. Don't choose a pattern with highly contrasting lights and darks. Think: seamless, open, flowing, calm. Use larger tiles on the floor and choose a grout color that matches the tile color as closely as possible so the grout lines dissapear.
# 5) Put the largest mirror possible up on the wall. This will really open the space and light will reflect everywhere.
# 6) Choose lighter, calmer tones and color palates so it doesn't feel stuffy and dark
# 7) If possible, go with a pedestal or floating sink, so you can open up as much floor space as possible. If your bathroom is really small (like new york apartment style) get a smaller sink or even a corner sink.