One technique I've found pretty useful for keeping my days on track is using a timetable. That is, dividing my entire day up into calendar entries describing what I should be doing at any given moment. I haven't really seen much of the humble timetable since I left university, but I think there are a few qualities that make it useful for creative work.
A timetable might sound overbearing, but it's important to remember that, like any measurement or management tool, it's only as draconian as its consequences. In my use, I don't find much benefit in getting upset when something happens and throws the timetable out. Rather, I adjust the timetable so that it makes sense for the rest of the day and use it as an opportunity to think about whether that adjustment was a good thing.
In fact, I'd say that's the main benefit of having a timetable. Without one, your day can slip arbitrarily and you don't even notice. Someone calls and you lose fifteen minutes. You get sidetracked by some interesting website and half an hour's gone. Suddenly the day's over and you're surprised to discover you've done half as much as you expected. With one, you not only notice the slippage, you can correct it by making changes to the rest of the day.
The other main benefit is that it forces you to put a bit more effort into planning your day. My default plan looks roughly like "do things until I'm done", which has the benefit of simplicity but not much else. Explicitly blocking it out makes it obvious when there aren't enough hours to do everything you were planning, particularly on days where other commitments intrude. It also makes it much clear when you're not leaving yourself enough time for breaks, exercise or fun.
Basically the only downside is having to remember to put aside time to do it, which is the kind of problem you can solve with the judicious application of even more timetabling.