I've been working from home for the last 13 years and I currently work for a company where everyone works from home. I'm not so sure that you need a door to keep your family out as to help separate your work life from your home life. The hardest thing I deal with is leaving work at the end of the work day. It's far too easy to slip back into the office to take care of that one little thing. This leads to burn out and should be avoided at all cost.
Set a schedule and a routine then keep to them. This should include a dedicated work space that you use for nothing other than work. That seems silly but it really helps maintain discipline. For example, I take my laptop to the couch at the end of the day for personal work and net surfing. I also found that I had to turn my desk away from the beautiful view outside my office to get any work done. I have a fixed schedule and though I need have to, I get approval from both my supervisor and my wife to work outside that schedule.
There is no reason to be in-accessible to your family but at the same time, they need to understand that you're at work. I've been in many meetings where a small voice is heard asking permission to go to a friends house. While this is never a problem, hearing a boisterous family clamoring in the background during a conference call is extremely disruptive. Likewise, slipping up stairs to change the laundry is probably not a problem though slipping out to mow the lawn is. Ask yourself before doing something personal, "Would this pass muster if you were in an office with other co-workers?"
Finally, the most important part of working from home is communication. Get yourself a high quality wireless headset and no-cost/low-cost calling plan. If you're not on the phone for as much as half you day then your out of touch with what's happening in the rest of the company. That voice line can be either POTS or VOIP but it should be dedicated to your work. Trust me on this, you don't want your teen daughter throwing a tantrum on the extension because she can't call her friend while you're in a conference call with a client. I find that a Plantronics Calisto Pro over a Vonage line gives me good quality so long as I'm not presenting. If I am presenting then I'll use my cell phone so VOIP does not compete with the presentation for bandwidth. Speaking of bandwidth, don't skimp here. My only choice is Time Warner which has an unpublished habit of dropping my bandwidth to 128K just when I need it most. When I complain they refer me to their Terms of Service regarding peer to peer file sharing. It's frustrating because none of my network traffic is peer to peer file sharing. I suspect the limit is triggered by upstream bandwidth use exceeding some parameter but can't prove it. Their best idea is to refer me to their "Business Class" service which gets me over priced hosting I don't need.
Instant messaging apps are also a must for the quick questions and should get you immediate access to anyone in your company. Think if it as the equivalent to shouting over the cubicle wall or walking down the hall to poke your head into someone's office. We found that no one IM service worked reliably, not even our own dedicated LINQ or XMPP server proved to be reliable enough. We currently distribute a list of employees and their IM accounts on several well known services. I am accessible on MSN, Skype, and Google Chat while at work. The redundancy means I can still contact someone if one service starts acting up. Unfortunately, it also means I'll have two or three messages waiting when I come back from changing the laundry.