If you are thinking of buying a decorative rug either for your wall or floor, here are just a few things you might want to consider.
As colour takes on the most fundamental role in any decor, this should be your highest priority when choosing a rug for your home. If a room's existing colour scheme is very bright and fussy the rug needs to be a controlling natural colour / design. On the other had, if you already have a natural scheme, a nice bright splash of colour and striking design will really make it the centre piece of any room. Sometimes a rug should enhance the existing decor and not stand out, using an already dominant or accent colouration will have this effect.
I've written a previous post about how using colour in a room affects you mood, which might be helpful in deciding your colours:
You might also want to consider how much wear and tear the rug will go through, if you have children or pets a darker and intricate patterned rug will fare far better over time. Just remember to make sure any patterns on the rug do not clash with those on the wall-paper or other soft-furnishings.
You might think that you can just go out and buy a rug without giving too much thought into the size and where exactly it fits into your room's layout. This really depends on the purpose of the rug and which room it will belong. If you are planning on the rug covering the majority of the room then there should be the same amount of floor space on each side, even if the room is an irregular shape then it still needs to have some sort of symmetry.
There are some basic rules when adding rugs to certain rooms, take the lounge for example. If the rug is going to sit under a coffee table then the entire table should be on the run and all pieces of furniture set back from it at equal distances. You should always consider where a rug will be placed in a room in relation to everything else, as long as there symmetry generally you will be ok. If you find it hard visualising where the rug will site, use some masking tape to define the space and play around until you are comfortable with the position.
Just remember that there are a huge variety of styles, and sizes to fit any decor or budget. So make sure you find the right one before buying.