Every January I put together my writing plan for the year and fill out my desk planner. I start by marking off birthdays, holidays, friends visiting, family plans and personal stuff. They have nothing to do with writing, but they still come first.
Contracted work comes next, and I block off an appropriate amount of time for writing and editing each book I've sold. Work not under contract but that I'm fairly sure I will sell and have to write this year (such as future novels for my active series) gets the next chunk of my time.
From that point, the remainder of the year is open for on-spec work, checking on open subs, writing freebies and special or private projects that may or may not involve enriching uranium. In some cases this is easy to schedule, like the free stories I want to post during the lag time between releases, or finishing projects related to particular dates or holidays, like the virtual workshops I have over the summer.
Once all that is penciled onto the planner, I look at the year and start shifting things that conflict or collide with family stuff, deadlines, release dates and that vaguely familiar concept I think I used to do called my annual vacation. Occasionally when I've stacked the schedule too heavily I'll cross out the non-paying projects to give myself a little breathing room.
The planner then goes in my office, where it sits open to the current date on my desk and waits for me to check it every morning when I write my private journal entries, letters, pay bills, etc. It also comes in handy when someone from New York calls and asks, "Can you do this by this date?"
I've only misplaced my planner once, during a household move, but after witnessing my reaction my guy and the kids searched ceaselessly until they found it. My guy then offered to chain it to my wrist or nail it to the top of my desk. All that because I shrieked a few times, tore out a little hair and kicked a few boxes into orbit. What can I say? I'm very attached to that planner.
Planning an entire writing year in advance won't work for everyone, especially writers who need more spontaneity to be creative, but it keeps me happy and I feel better knowing what my workload is. After January 1st most book stores have 2008 calendars and planners marked down 50-75%, so if you'd like to do something similar you can get a nice one for a couple of bucks.
One more plannering secret: if you can manage it, leave a few days to one week blank every quarter to create some space for the unexpected. I always end up needing the extra time for something, and if it's there already, I can add in a job or project without piling up more demands on my prescheduled time.