I've used the pomodoro + Spaces + single monitor technique for a while. I actually took things a bit farther and now keep my laptop on a stool to the left of my desk (I do my main work on a desktop). Instead of having a separate Space for non-work stuff (chat, email, casual browsing), I keep it all on the laptop. I only use it during pomodoro breaks and I have the screen go to sleep after only a couple minutes of inactivity so it's not distracting when the breaks are over.

I've found that since I have to perform the physical act of swiveling my chair to comfortably use the laptop, it is much less tempting to get distracted. It was too easy to "just check my email really quick..." or "just respond to this one instant message..." when it was only a quick Spaces shortcut away.