From The Pmarca Guide to Personal Productivity (click the link, it's interesting):
[R]efuse to commit to meetings, appointments, or activities at any set time in any future day.
As a result, you can always work on whatever is most important or most interesting, at any time.
Want to spend all day writing a research report? Do it!
Want to spend all day coding? Do it!
Want to spend all day at the cafe down the street reading a book on personal productivity? Do it!
When someone emails or calls to say, “Let's meet on Tuesday at 3”, the appropriate response is: “I'm not keeping a schedule for 2007, so I can't commit to that, but give me a call on Tuesday at 2:45 and if I'm available, I'll meet with you.”
Or, if it's important, say, “You know what, let's meet right now.”
Clearly this only works if you can get away with it. If you have a structured job, a structured job environment, or you're a CEO, it will be hard to pull off.
But if you can do it, it's really liberating, and will lead to far higher productivity than almost any other tactic you can try.
There's more; if this bit appeals to you at all, the whole thing is worth reading.
By odd coincidence, pmarca is Marc Andreessen, cofounder of Netscape.
Credit: Marginal Revolution.