Though a bit narrow, I have found desktop (menubar) timers with ticking in the background to help a lot. As a matter of fact, I see mine counting down from the 5 min I allocated to check out YC and I can hear the ticking. What has surprised me most is that I haven't yet developed the ability to tune it out - so I always hear it. It also acts like a metronome and I believe it makes me work a bit faster.