The Pomodoro Technique, created by Italian Francesco Cirillo, suggests that you divide your working time into blocks of 25 minutes (when you work), followed by a 5 minute break (when you rest your mind, catch up on email etc.).
The idea is that it helps to alternate work and breaks, that it's hard to focus beyond 30 mins at a time, and that you can get more productivity by not mixing work & recreation (which has become quite difficult given the environments most of us operate under these days).
Some people who've tried it have opted to extend the duration of a period from 25 to 45, followed by a 15 minute break (essentially following a 60 min cycle, rather than a 30 min cycle). Which makes sense since switching in and out of context has to be budgeted for.
Others have just ignored it and stuck to their own messy but tried ways of operating. This model is hard to pull off if you aren't in too much control of your own work time: if you are interrupted by peers, managers, emails, calls, telemarketers, tweets etc. But we think it's worth a try.
You can use a simple alarm clock or set your mobile phone's alarm to do this. You even get specialised apps/browser extensions to manage this (we've tried them - mostly overkill and a reason to procrastinate).
Oh, and what does a tomato have to do with all this? "Pomodoro" means "tomato" in Italian. Apparently, Cirillo, used a timer in the shape of a tomato to try this method.