I'm working hard on getting as much detail and detritus out of my brain and into my organizing system as I can. My calendar/to-do list combo (for me, on the iPhone) is working well enough that my mind is clear enough to detect other, less obvious details I've been forgetting for a while.
Here are a few things I've recently moved off my plate by making a note in my calendar and asking it to remind me when the time comes:
Gift ideas and purchases: You see the perfect gift for someone, you buy it, you store it in "a safe place," you feel smug that you've taken care of gift-buying so early, and then you forget about it and waste money on a duplicate gift. Set a reminder for around the time you want to give the gift with the name of the gift, the recipient, and where it's hidden.
Delegated tasks: You're so excited to get something off your to-do list (and onto someone else's) that you forget to follow up to find out if the task was actually finished. For example, set a reminder to check your credit card statement for returned item credits.
Irregular paychecks or reimbursements: If you're a freelancer, you don't always know when to expect payment (and the folks who owe you money aren't necessarily going to remind you). Set up a reminder to check your mailbox around the time your next paycheck is due.
Obscure home maintenance chores: Changing your furnace filter (every three months!), cleaning your refrigerator coils (every six months), cleaning the dryer vent (once a year), etc.
Warrantee and service expirations: I have a Sears service contract for my washer and dryer which includes one preventive maintenance visit every year. My contract expires next week. Good thing I had a reminder set up — I would have missed out on service I had already paid for. (And may none of us ever know the disaster that is a broken washer or dryer.)
Automatic renewals: I just got burned on this one. I forgot to set up a reminder about an online subscription that renewed automatically, and then wondered about the weird charge on my credit card. (They were willing to credit me, but I had to spend time on the phone.) Check your accounts and donations, then set up reminders for a week before the renewal dates so you can make an informed decision about whether you want to keep them going.
Do you have any "sneaky" reminders to share?