Gosh, its a tie between several things. Most of them came from this board (ditto Sue) but one other really good piece came from someone else, and I don't even know who anymore - when wee difficult child was little and I couldn't even take him to the grocery store, someone said they stopped dragging their child and just said "I'm going to the store" and took off walking (with a attentive eye behind them, of course). And their child always followed, even into that wretched world called the super market.
And when I tried it, SO DID MINE! That person gave me the ability to buy groceries, and it was SO COOL!
(and actually that little tidbit has come into play many, many times more. Just do it. Many times, difficult child will follow.)