Depending on its purpose, aim to purchase a rug that is larger than the furniture that will be placed over it. Try not to lay a rug on its own, unless it is to cover a large expanse of open floor space (see previous photo for a good example of how this can work).
For dining areas, pick a rug that covers the full size of the table plus allows for chair movement as this will prevent scuffing floorboards or tiles. In the lounge, you can afford to place a larger rug under multiple items of furniture. Partially or fully place your sofa and coffee table over the rug and you will now have a defined space with a point of interest.
Finally, always purchase a good quality anti-slip mat to keep your rugs nice and flat and prevent any unnecessary slipping. I normally pick mine up from a local carpet store, but you can also find them in most stores that stock rugs.