A tickler file is a way to write notes to your future self. At its simplest level, it is a calendar where you write future reminders to yourself on the appropriate dates. For instance, mine will have 1-year, 6-month, and 90-day reminder notes for when a community is coming up for refinancing. I cheerfully mix personal, social, and business: anniversary and birth dates are sprinkled in with meetings, project due dates, and business trips.

There are many ways to keep a tickler file including electronically. The key thing is to
1.  keep it all in one place
2.  consult it regularly
3.  put relevant items on it IMMEDIATELY and RELIGIOUSLY
4.  add pre-reminders for events that require preparation

Wikipedia: A tickler file is a collection of date-labeled file folders organized in a way that allows time-sensitive documents to be filed according to the future date on which each document needs action. Documents within the folders of a tickler file can be to-do lists, pending bills, unpaid invoices, travel tickets, hotel reservations, meeting information, birthday reminders, coupons, claim tickets, call-back notes, follow-up reminders, maintenance reminders, or any other papers that require future action. Each day, the folder having the current date is retrieved from the tickler file so that any documents within it may be acted on. Essentially, a tickler file provides a way to send a reminder to oneself in the future—”tickling” one’s memory.