Post a Sign
This is a critical tip for parents trying to work from home. Whether you are working in an official-looking home office or a milk crate desk in the corner of your bedroom, make sure that there is a door between you and your treasured distractions (er, children). If your kids are older, this door is a great place to post your daily work schedule along with a number they can dial to reach customer service, i.e., your spouse's cell phone.
If you have younger children, you need to post a sign that can be easily interpreted by a toddler to mean, "I love you, but please go bother daddy." Try a large "X" or a color-coded system like a construction-paper stoplight — green means "go on in," yellow means "enter only if you are bleeding/vomiting," and red means "there is a monster behind this door."