Choosing a soap dish is mostly an aesthetic decision; whatever looks the best with the rest of your bathroom fittings will usually be what you end up going with. You may also choose not to have a protruding or freestanding dish and opt for a recess instead. In either case, there are practical considerations you should take into account when deciding where to place it.
Shapes and materials
A soap dish can be made of any number of different materials – provided that it's watertight and easily cleaned, the material is not a major concern. Common materials range from a simple plastic tub to ornate tempered glass creations which would be as comfortable displaying jewellery as they are your soap. They can be shaped like racks with no solid base, like wire baskets, pedestals, or any number of other designs. They're generally either wall mounted or freestanding.
Recessed soap dishes are either built into the wall, usually when the bathroom is being made, or moulded directly into the tub or basin in the case of vanities and bathtubs. Wall-based soap recesses are often simply moulded in and tiled with the same tiles covering the walls, but some ornate, prefabricated types are available as well. They can have the advantage of being able to provide a covered 'shelter' for your soap, but are obviously permanent fixtures and cannot easily be relocated or replaced once they're installed.
Where to put your soap dish
The main thing to remember about locating your soap dish or recess is that if it's anywhere near a direct line of water (don’t forget to account for splashback from someone using the shower) your soap is likely to constantly disintegrate into a gooey mess. You’ll need to keep it convenient and handy, but well away from running water wherever possible.
Choosing the soap dish is one of the few completely aesthetic choices you’ll have to make when designing your bathroom - don't be afraid to be a little creative!