Trulia had an article posted with tips for making your home move in ready before you plan to sell. Here is what they say about why and when you should start:
The moment you even begin to think of putting your home up for sale is the time to start thinking like a buyer.
Why?
Because thinking like a buyer will help you make your home move-in ready for just the right buyer (or maybe even a bevy of bidding buyers). The less they think they have to change, the shorter the distance will be between the viewing, their offer, and the day you say, “Sold.”
You can see the full article for the 7 ways they recommend to make your home move in ready and more attractive to potential buyers.