If you have an idea of a look that you want to recreate, ie Mediterranean. Start looking for examples in books, magazines, on TV and pick out the things that you think highlight that period or style.

Focus on these and see wether they will fit into your space. Sometimes the structure / canvas we have to work with will not be capable of taking on the new style you desire.

You have to be practical. For example you want a French provincial look, weathered and rustic, but your home is contemporary with lots of large expanses of glazing on the walls, minimalist lines and form.

This would be very hard to pull off. You need to consider the bones of the room to start with. This also relates to the whole home.

I do not advise having different themes or periods for different rooms in the same house. This can make your house very disjointed and unpleasant on your eye.

Not to mention the house losing the natural flow and harmony that can be achieved by making one room flow into another. You can have similar themes that relate but dramatic changes are not recommended.

There are many ways to use styles and periods in your schemes.

If you have period furniture you can use the colors of that era in your decoration to enhance the furniture and create your style. If you have 20th Century contemporary furniture you can use it as the decoration and keep the interior simple therefore focusing on your designer pieces of furniture. You can combine different periods, this is called eclectic. It is important to keep the background simple so that your collections of design ware are the focus of the space.

Another thing to consider is that you have to love what you are going to do, don't just do it because the look is "in" at the moment. You have to live in the space, before making your decisions, remember that!

To conclude, if you have your heart set on a theme, do some serious investigating and research before you embark on fulfilling it. You never know, with so many options out there you may find something that you like more and suits your home better.