I dont know if it will be of any help to you, but for myself, I find that I actually get more writting, and other creative things done, when I only have a certain amount of time I can give to them. For example, at one of my last jobs, I found that I got excited and wrote ALOT of partial scenes and story boarding for a game i was working on, when I could sneak 30 mins or so of it in, sitting in my office waiting for clock out time to go home. In fact it even motivated me to work harder to get all the things i needed to get done for the job that day done, to get those precious few minutes. By contrast when I was in between jobs, with alot of hours to fill, I just didnt feel motivated to be creative. I had all the time, but couldnt muster up any desire.
Of course, that could just be a quirk particular to myself. I havnt had a chance to discuss with alot of other writers how they get their motivation up and get productive! Still I hope that sharing it helps one way or another. If nothing else at least you know your not the only one who has trouble in that regard.