And I hate to be the one to have to break plans, even if it IS because of my poor schedule keeping.
The solution? Use the Google calendar feature and the calendar app on my iPhone. It's as simple as that. Really. I sit in front of a computer all day and I am rarely without my iPhone. There's no excuse for not being able to input dates/times/plans right when they're being made.
By doing so, I can save myself the embarrassment of overbooking and then having to admit to my failure.