My basic rule of thumb is:

• Use the greenery first. Spinach, watercress, mixed salad leaves, Asian greens, fresh herbs, anything with a soft green leaf will start to look quite sorry after a couple of days. So use these in your meals first.
• Always choose some long lasters. Potatoes, sweet potato, pumpkin, onions, garlic, leeks, cabbage, even beetroot and sometimes carrots will all survive a week. Some will be good for longer than that. So I always buy a couple of these and plan to use them at the end of the week.
• Use your judgement with the rest. The longevity of everything else lies somewhere between these two extremes, so plan to use them mid week, once the greens have gone.
• It helps to be a bit flexible. If something in you fridge is starting to lose freshness then use it up.