I also both close and lock my door. The problem stems from the fact that while my wife intellectually understands I'm at work, she has trouble accepting it on a more emotional level. When kids are acting up, or she is having a rough day she tends to get resentful that I get to "relax" in my office while she deals with the various disasters. Actually one of the biggest challenges is meals. Do you take your lunch in your office? Eat with the family? Eating with the family sounds great, but it can really derail your train of thought. The only thing that seems to work for me is to be really strict about it, and deal with any resentment head on at the end of the day.