how to pack your shopping bag

David Wolstencroft describes himself as a supermarket expert and scriptwriter. Here he gives us the benefit of his shopping knowledge, with tips on how to avoid your lettuces becoming limp and your eggs scrambled

Step one: preparation. In effect, your perfect bag actually begins to take shape long before you reach the checkout. They didn't start building Canary Wharf by turning up on the first day with the carpeting. Similarly, you shouldn't throw your beans on top of your sprouts unless you have a very good reason for doing so.

Plan your trolley space. Foresee potential bottlenecks. Place bulky items to the sides, and growing herbs in the front.

After planning comes foundation, as any engineer worth their BSc will tell you. However, in the cut-and-thrust of bag construction, a supporting wall is often required before the foundations are created. Namely, cereals, harder breads and any other cardboard rectangles which should go across the bottom to build your base, to allow for more substantial and fruitful lower levels - beans, tinned tomatoes, jars of frozen wasps in brine. The key here is to create what professionals term a solid "grocery cement", but without unnecessary weight.

Now, you begin to build. The middle sections are the most important by far. It is, quite literally, make or break. The trick is to offer both substance and protection - carrot cakes, lettuce, freshly baked bagels, freshly squeezed juices are favourites here.

The major mistake made in this vital central section is to overlook the eggs. Eggs have always been problematic for the inexperienced bagger. But for the professional, they are as easy as pie (indeed, often easier if the pie is fresh).

When purchased in cardboard casings, eggs are simply another functional filler - and what's more, they're a shapely, well-protected and structurally sound invention of the truly creative bagger. An explosion of texture, gentle as broccoli, broad enough to give shape, narrow enough to permit handle closure (except when dealing with freshly-baked pizzas, when you may just want to have a completely separate bag).

The perfect bag has balance, shape and structural integrity - indeed, at the international level it should be able to withstand an earthquake and win the Turner Prize at the same time. Of course, few of us will ever attain the true bagging greatness of the experienced supermarket professional. But it is important that we learn.

So for once, when you've returned home from the supermarket, leave your bags on the counter for a while. Look at them closely. Learn to appreciate, perhaps even to love. And remember, if Inigo Jones were alive today, he'd be working in Tesco's.

'Shelf Life', David Wolstencroft's new comedy (starring Tony Gardner, above), can be seen at the Gilded Balloon II, Stepping Stones Theatre, Edinburgh (0131-225 6520), 1pm to 2 Sept