Every job has a uniform. Doctors have their white coats; business executives have their power suits; and freelance writers have their sweatpants (only sort of kidding). Uniforms serve two purposes: (1) they communicate to the outside world a person's position, stature and function, and (2) they reinforce those same messages to the person wearing the uniform. When a doctor puts on her whites, she puts on the mantle of the knowledgeable and capable healer. Without it, she's just some lady gagging you with a Popsicle stick.
When you work from home, resist the urge to remain in your pajamas. True, no one but the UPS guy is likely to see you in your polar bear-themed sleepwear, but it's hard to take yourself seriously as a worker when you look unemployed. Dressing in your normal work uniform — even if that uniform is a T-shirt and jeans — reinforces the idea that you are actually working. If wearing a tie or pencil skirt really gets your juices flowing, go for it. Just don't expect the cat to be impressed.