I tape a sign to the door if I have a conference call so they (wife and kids) know not to disturb me.
Lock the door if you get bothered. Plus have some music handy in case there is a distracting noise (trash truck, kids, annoying spouse). It helps drown out some distractions.
Also keep distractions out of the office (ie. games, keyboards, guitars, etc).
I take frequent breaks using WorkRave at least every 15 mins so I remember to sit up, move and relax my eyes.