Best way to exit a buy and hold strategy?

Let's say you've been diligently investing some money in an index fund every month, and now you want to pull it out so you can retire. If you only pull out what you need every month, you run the risk of a market crash wiping out a large chunk of your net worth when you're relying on it for income, forcing you to realize significant losses. If you sell all at once, though, you're giving up any future returns you could be making on your extra money. (There's also the risk of retiring during a market crash, which would also force you to realize significant losses.) So, what's the best way to preserve as much of your wealth as you can when you need it, while still compounding the money you don't need?