As mentioned, one part of creating a work at home environment is making sure that you have a space dedicated to working at home. You will find that you are much more productive when you are working at a desk, than you would be when you are working it a more relaxed area like the coffee table.
Another part of creating a work at home environment is having the proper support from your family to work at home. While I have been blessed with a family that understands what I do on the computer is actually work, I have heard from many others that their family doesn't consider what they do to be actual work. They are just 'playing' on the computer. This in turn can lead to a negative at home environment that causes you to be less productive and unhappy.
One last way to create a positive work environment is to keep your home a calm and organized area. I have found that I feel my best when working when the room around me is organized. Being at home, it is often possible to let the laundry pile up or the dishes sit a bit too long in the sink. When it gets like that, it nags in the back of my head making me feel like I don’t have enough time in the day to get everything done. By making sure the house is organized each night, I can allow myself to relax a bit and not get overwhelmed.