A couple of thoughts have helped me (I love to hit 3 and 4 irons):

Sam Snead said his first movement when starting the swing was forward
Jack Nicklaus never agreed with the theory it is fine/beneficial to transfer weight backward when starting the backswing. Nicklaus' head did not move during the swing.
Johnny Miller said he would imagine pushing a nail into the ground with the big toe of his front foot when starting the swing (this helped more than anything)
Tom Watson has said many times he likes to warm up with a three iron because if he hits it great, great, if he doesn't, no biggie because you're not supposed to hit a 3 iron great.

And there is the beauty of it. My favorite shots of all time have been with a three iron. It can be terribly frustrating, but there is nothing like the feeling of clean contact, hearing the sound similar to a match strike, and looking up to see the ball take off for the green from beyond 200 yards.