After overeating for the past two months, people are starting to think about slimming down for summer. Others are cooking more to save money after spending too much over the holidays. If this is you, planning is key. Don't depend on your willpower to make good decisions all the time. It is far too easy to order pizza or pick up a sandwich from the corner. Instead, use your willpower when you have it to plan ahead for the times you don't. Here is what I do.   Shop first,…