If you find yourself vowing to do some writing but then reach the end of the day not really sure where the time went, I strongly advise keeping a time diary for a few days. Make sure you pick days which are fairly typical in terms of your planned schedule.
Write down EVERYTHING you do during the day, along with a time stamp. Yes, everything. Don't be embarrassed -- you're going to throw this out when you're finished, so no one else will see it.
I tried this and was horrified at how much time I was wasting or frittering away on non-work activities. The exercise really makes you more conscious of how you're spending your time.
These days I still try to keep a work diary, but not nearly so detailed...just enough to help keep me on track.
Has anyone else tried this exercise?