Somewhat related: I've been having a lot more success keeping myself focused since I started using the fullscreen mode of iTerm to do my writing. There's no clock in the upper right corner of the screen to draw constant glances and I find myself less likely to randomly switch to other apps to check on my feeds, emails, etc.

It's even slightly helpful for getting myself started, since all I have to do is switch to iTerm and take a look at my notes, at which point I often just pick up where I left off and can continue undistracted. (E.g. this morning a quick app switch to remind myself where I was became a solid 40-minute session -- I was putting off breakfast to get stuff done!)

Along the same lines, earplugs (or in-ear phones if you work well with music) help too.