Hi there
Yes, slouchy days are not good. Even though some days will naturally be more productive than others, you don't want have too many regular slack days. Designate your days - either study, do admin, or have a day off etc. As others have said, treat your PhD like a job. While some days in the paid workforce do bo by without a lot being achieved, if you were in a regular workplace, you wouldn't slacken off would you? Force yourself. Have breaks, but plan your days, allocate tasks, and only be slack when you've earned a short break. I let myself goof off every couple of hours on the net, just for a bit, and rationalise that if I was in an office, I'd have the occasional conversation with a colleague, so that's OK.
Right, that's me being slack, back to writing!