37signals - Getting Real: Alone Time:
"Set up a rule at work: Make half the day alone time. From 10am-2pm, no one can talk to one another (except during lunch). Or make the first or the last half of the day the alone time period. Just make sure this period is contiguous in order to avoid productivity-killing interruptions.
A successful alone time period means letting go of communication addiction. During alone time, give up instant messenging, phone calls, and meetings. Avoid any email thread that's going to require an immediate response. Just shut up and get to work."