The key to being reasonably productive is to discover what kind of person you are. We are born with certain personality traits that we can't really modify much. Some people are able do three or four different things a day. I tried. I failed. I am a one-main-task-a-day person.
This means that, ideally, I try to assign one core activity to each day of the week. If I'm writing, I mostly just write. If I'm doing university-related things —lecturing, grading, preparing for classes, meetings— I mostly do that.
My current week looks like this:
MONDAY: Preparing for classes or writingI usually begin work at 9 a.m. and stop either at 3 p.m. or at 5 p.m., depending on the day. Every hour or two —this is flexible, I must admit— I allow myself some goofing around in social media. This is when you may see me tweeting.
TUESDAY: Teaching and meetings
THURSDAY: Teaching and meetings
FRIDAY: Consulting/traveling or writing
SATURDAY: Freelancing or writing
SUNDAY: Family day
If there's an activity that needs to fit into one specific day —say a phone call with a colleague, a meeting, or filling out some paperwork on Wednesday, when I should be writing— I treat it as a pause, like if I were on social media.
If this secondary activity takes longer than 15 minutes, I make up the extra time extending my work day. I do the same if I get carried away on Twitter, something that recently has happened too often.
I try to reserve at least two or three hours a day to reading. I read two print newspapers and then my RSS and Twitter feeds during breakfast. This usually takes one hour or a bit more. I read books in the late afternoon or evening.