I go on and on here about fit when it comes to your clothes.
And, God knows, I'm still doing the "Do as I say, not as I do" thing in my closet - through I have been cracking down! (Just ask the wonderful seamstress who has been altering practically everything I've purchased lately!)
But it really is one of the best things you can do to improve your look - make sure your clothes really do fit you well!
Over at 24-7 Style another good post on determining whether or not your stuff actually fits...
Stop! Does the Shoe Really Fit? A Sizing Check List