Start a house diary- it may sound like a silly little tip, but it will sure make your life easier in the long run. You can use a real diary, or something digital, like Evernote.
Write down information pertaining to home improvements in your house diary- things like renovations, when they were done, who they were completed by, warranty information, and the total cost invested.
Make note of new appliance purchases, repairs, and so on.
Keep track of when your septic was last pumped, if applicable, and also things like water test results. Dates are easy to forget as time goes on.
A house diary is also a great spot to record things like when a room was painted, and what paint color/ formula was used.
Do you love to garden? Make note in your house diary of what that shrub or perennial actually is, and any specific information related to care that may have been on the tag.
When the time comes to sell your house, you will be relieved to find that all of this pertinent information is right at your fingertips!