Chris Worden from Worden Farm taught me how to do this. To make bigger and better Brussels sprouts. When the Brussels sprouts are about the size you see in the photo below, strip off the bottom leaves. Keep about 12" or so of leaves on the top to keep the plant going. You can cook these leaves like collard greens and no one will know the difference!
Then break off the top crown of the plant. You can eat that too. Soon you will will have nice, plump Brussels sprouts.