By all means buy the next size down, so a size 14, as motivation to get down to that. Then once you are a 14, buy some things in size 12. Finally when you are size 12, buy somethings in size 10. That way you aren't waiting for ages and ages to try and get down to the things you really want to wear.
But if you buy something at size 10 when you are size 16, chances are you've got a long wait, and you might actually find yourself getting fed up and depressed that it's taking so long.