There is a plethora of different looks for your curtains but you should always be generous with the fabric that you use.
Whatever the style of the room, the curtains should comfortably cover the window; if the fabric has to be pulled quite flat to meet in the middle they will always look cheap, cheap, cheap. Allow a minimum of twice the width of the window and a maximum of three when you buy it.
Those clever designs that seem effortlessly to pool on the floor can look amazing. This look works better with light fabric that flows and drapes easily; anything too heavy will bulk up on the floor into a heavy pile rather than sink down into a light pool.
Never buy fabric for curtains based on a tiny swatch. You will have no idea about how a print will look when it is made up into curtains if you have only seen a small square of the design. It would be a bit like buying a still-life when the artist has only sketched a single apple: how are you going to know what the overall finished piece will look like? The best way to get an idea when you are buying fabric for this job is to grab the roll off the shelf in the shop and pull out a metre or two so that you can see the complete repeat of a design.
Depending on the place that you are decorating, look to lovely shears, muslins and voiles for rooms where you want the light to flood in. They have a luxurious air when allowed to drape in generous swathes. One trick to add a more formal touch is to hang them behind a pelmet which is covered in a contrasting fabric. Choose damasks, heavy linen and textured silk when you want to make more of a statement with the drapes when they are closed. There is no doubt that you will have already made a decision about your colour scheme when you come to choose the fabric but think about the different effects that patterns can achieve. If you want to create the illusion of extra height, then opt for a design with a vertical stripe. If you want to add width, then pick a material with a horizontal design.