Honestly, this is something we should do more often too.
Book in advance, but make sure that your flight or hotel has a clear, no cost cancellation policy.
Be sure to mark the last day you can cancel by on your calendar, and set a reminder for a day or two before.
If your plans change, or if you find a better deal before the cancellation date, you can cancel without penalty. If your plans don’t change, you’ll have grabbed a good early booking discount