1. The first thing you need to do is to put some art on the walls. Don't use old posters you've had around the house. Find some fresh, new, inexpensive art posters and use an old frame you have. Better yet, go to Michaels, buy a canvas with one of their weekly coupons and paint your own art with a pack of acrylics. (I've got my own art all over my work areas.) Take a look at this Curbly post on DIY Font Wall Art.
2. Take everything out of the room and start fresh. I always struggle with moving things around in a tight room, but I don't have so much trouble when I start with an empty room and add only necessary pieces, one at a time. If you've got leftover furniture, consider NOT stuffing it back into the office.
3. Remove everything from the room and closet that is a homey/family item. Even purge the closet of blankets or extra clothes. Make the closet a storage spot for office supplies. You need to feel like you're at work, not up in the spare bedroom.
4. Since you've moved everything out, go through every single thing in that desk and file cabinet and shred what you don't need. This is the time to get organized. One of my all time favorite things is to go buy new supplies. Get color coded folders, a nice new calendar, but keep things simple. Simple organization is the key to staying organized year round.
5. Now the tough part--what do you do at the windows? Personally, I don't feel like fabric drapes or curtains say 'office'. I would try to save a little and go over to Lowe's or Home Depot and get some shutters made for the windows, or you could apply window decals that are attractive to look at and they don't block the light. If you prefer curtains--go for it! Make them or buy them, but make sure they are well made, especially if you'll have clients in.
6. Get a live plant that will survive and look great, or if you know you'll kill it, arrange your desk to look outside. You'll love to see something green and alive right there at work and it will make that creative thinking view so much nicer.
7. If you're working with all sorts of odd pieces of furniture like so many of us do, consider hauling everything out to the garage and painting all the furniture white. It will give you a look of uniformity and it will look great against walls painted in a darker shade.
Love where you work!