More people should read The People's Republic of Walmart. It's such a great book on this topic, and it completely destroyed any ideas I had that markets might be more efficient than planning. I do think there needs to be a well thought out pathway from markets to planning in order to ensure a smooth transition, but that's an entirely different discussion.

EDIT: This essay by Sam Gindin also does a pretty good job (in my opinion) of laying out what economic planning in a modern socialist system might look like, including the pathway to a layered planning system from a market system.