I will be honest this is one area that I am still working on organizing better!
I have some folders by year of activities done. I do the same on my home computer.
On my home computer I save my newsletters by the month and date (that Monday) and I save my lesson plans by year, month, date.
For the next school year I edit the previous year's and resave it as the date for the new year, month, date and I do the same with the newsletters.
I should probably save the activities done by month as my themes do not change too much, but I just have them all in one folder and I have a hard copy and I look that up in my school year folders to find the activities.
Check out more great digital file organization ideas from PreK Partner and Learning and Teaching with Preschoolers.