One pretty common technique that I use when the going gets tough is to use a timer. I set it for an hour, and I have to write for that long. If I get up to make tea, answer doors, etc., I hit the pause button (it's electronic) so the hour doesn't get frittered away.

I haven't really had to do that much though since I took up afternoons on AIM with Jessica (http://www.livejournal.com/users/jess_ka/)!