You know that feeling - you just need something NOW! Urgency is one of those evolutionary feelings. If you feel something urgent, you will do almost everything in your power to accomplish it. Urgent also causes us to overspend on items of much less value to us. Marketers love urgent and do their utmost to cause a feeling of urgency in consumers, so goes the cliche "for a limited time only."
When you plan what you eat, you usually eat healthy. You know that you eat dinner, so you buy groceries beforehand and prepare a meal that's ready by dinner time.
But what if you're hungry now!?
If you're away from home and you're starving, you'll grab a bite to eat at a fast food joint - eating more than you need to and feeling lousy afterwards. To avoid this type of eating, some people eat several meals a day and keep healthy snacks in their car, office, etc.
Overspending when you feel an urgency to obtain something can be overcome in a similar manner. There will be instances when you'll need something immediately, so become intimately familiar with the shops in your neighbourhood. These are your "stashed snacks" and you'll know which one offers the best value. Are sets of screws cheaper at Home Depot or Walmart? Or does the Daisy Dollar sell them for the best price? Knowing where things are sold and for how much is knowing where to get the best value.
Secondly, be prepared for an urgent need right now. When going through your weekly flyers (what? you don't do this?) don't ask yourself "do I need this?" but instead "WILL I need this?" Moving the need to the future means being prepared for that need when it comes. If it's on sale, buy it now and keep the receipt. If you don't use, return it.