1. Allocate a time slot for yourself, within 1 - 2 hours. 

2. Make it clear that within this period of time, you will do absolutely nothing but the single task (which would be your most important task) in your mind. That means no phone calls (keep the phone someplace else), no Twitter, no wiki surfing, no Hacker News and no email, etc.

3. Do that single most important task. If you think it would take longer than 2 hours, break it into several parts where one part could be done in a single time slot.

4. Once finished, take a mini break to refresh your mind: take a shower, have a tea/coffee, workout, read some magazine or have a snack, etc; if you are still going strong (or, 'in the flow), keep going.

5. Rinse and repeat.

Using this method, I could easily get a productive 10-12 hours from a normal work day. Even when I'm travelling, I could still get some real work done. If you are one of the rare and smart readers of this blog, try it and let me know how that works out for you (or let me know if you have a better way).