Add in layers. This will give warmth and dimension to your window treatment design. This is also the difference between just throwing up curtains and really getting a designer touch in your home. This doesn't mean that you have to use a lot of heavy fabric. You could try a lighter plaid Roman shade next to floral curtains. This could be perfect in a kid's room or you could also use it as a country touch in a living room. Fabric is a great place to add color and pattern in the room because it's not permanent or expensive.
Experiment with tiebacks. This is a simple way that you can incorporate your overall theme. It adds elegance and can tie into other metallic finishes for a subtle touch, but it will help your space to feel finished. It's also functional. You can find some of these items that are like hooks that will help curtains stay open in the daytime. You can also add decorative tassels. Another option would be to install doorknobs or try making your own hooks or use decorative corbels. This will be very oversized and dramatic so it is perfect for an elegant window treatment.
Highlight the shape of your window. You can do this a few different ways. You may have to have window treatments made specifically for your windows. A lot of modern homes have windows with special challenges. They focus on unique shapes. You really need to consider which windows actually need treatments and which windows don't. If you have really high ceilings some of your upper windows might not need coverings. You could do a decorative stencil around the window to highlight the archway. Then you could just use plain curtains on the lower windows to save money but still bring in fabric and privacy.
Sometimes less is more. It's easy to go over the top with your window treatments. One of the best design ideas is to just use a little bit of restraint. This will actually make your room look more elegant and formal. It will be easier to clean and less expensive to install. You might just want to do the exact opposite of what is going on with the walls. If you have patterned wallpaper then try using a solid color drape. If you have a solid paint color then try a patterned fabric to spruce up a boring wall.
Change up the length. If you want to go with an elegant window treatment then try a super long curtain. If you want something less formal than just cover up whatever part of the window that you need to for privacy. This will allow light to stream in and also use less fabric. Café curtains are bistro or country feeling so it is perfect for a kitchen or dining room.
