Some of the principles of kitchen design can be carried over to bathroom design if you are going to have furniture or cabinets in your bathroom, so you should check through Tips for Kitchen Design as well as reading this.
Generally a bathroom will contain three or four main pieces of equipment: the bath, the shower, the WC and the wash basin. Most bathrooms are generally quite small so careful planning is essential to make sure you get the best use of space and fit everything in.
It is usually a good idea to take careful note of where the waste and water supply is in your existing bathroom unless you are going to completely remodel it so you can position the services around your design. This is not usually very practical because positions of drains are usually fixed or very costly to change.
Note that all the items will need to have water supplied to them and waste taken away from them. It is usually quite easy to redirect the water supply to where you want it but the waste is a different matter. There should be a 'fall' on the waste pipe so that the point at which it connects to the unit is higher than the point where it leaves the room. A steady shallow fall is best because it allows the waste to drain away at a steady rate. Note that some showers and baths may need to be raised off the floor on a well constructed platform to give enough 'fall' for the waste. It is not generally a good idea, unless there is no other option, to route waste pipes under floors and through floor joists.
Begin by positioning the WC because the location of the SVP (soil and vent pipe) is probably one thing you cannot change very much. Waste pipe extensions and coupling can give some flexibility in positioning but it is best to keep these to a minimum - remember that all pipe runs should be boxed in to give a neat and tidy appearance.