When it comes to the business calendars, I keep them all online. You do not really have a choice anymore, since fewer and fewer people actually carry their own personal calendars with them. While I have been an administrative assistant long enough to remember the days of a paper calendar fondly (and wish that my current employer would keep one in his back pocket instead of just calling me to pull it up online!) I have bowed to the inevitable and now keep all of our business calendars online.
However, I admit, I have a "dirty" little secret. It is one of those 18-month calendars that you get from the office supply store with 3 days on each page. It's about an inch thick, and I love it. Honestly, I don't think that I could keep track of everything without it. The virtual calendars are great, but my master calendar is still on paper, and I've found that everything runs smoother when I can just thumb through and find any information I need right away.
I still transfer all of the info to the online calendars, but this is my completely private master calendar. It lets me see how every little thing I know about the workday and my own personal life will work together to keep me productive (or not), and it also lets me put in notes about things to check on that I don't actually want on the official calendar.
For example, a few weeks ago the boss was having a serious tiff with his former wife. Of course, this was none of my business. However, it will impact his work schedule since they share custody of their kids and were squabbling over when he would see them for the holidays. I know he takes days with the kids off entirely, so I marked down on the calendar to check in with him in about two weeks to find out when he will be off. Given the stressful nature of their relationship, I know from experience he'll forget to tell me ahead of time - if he even knows until the last minute. That's the sort of thing a good assistant keeps up with, but never lets you know she's paying attention to.
My paper calendar is like my "little black book." It's got all my trade secrets and contact information that I want to keep quiet inside. I'll never put it online, and, except for here, I'll never tell...