God never meant the British to picnic, but we battle on. And battle on we must, regardless of the wind, rain and thick cloud that appears quicker than you can unfasten the leather straps on a picnic basket. Regardless of nettles and wasps' nests, of barbed wire and cow shit. We know that fresh air sharpens the appetite and lifts the spirits. Sunshine and a light breeze can make even a ham sandwich twice the meal it is indoors. In short, food tastes better outdoors. This alone should be reason enough for us to pack the car boot with French bread and bubbly cider. But there is more to it than that. The need to picnic is a part of our collective national soul and nothing, but nothing is going to stop us.

The perfect picnic, like the perfect Christmas, exists only in a far off corner of our mind; a retreat, an escape, a place to go in a daydream or on the shrink's couch. We never have, and almost certainly never will have, that idyllic outdoor meal on a checked rug by a babbling stream, because as they say, it's all in the mind. What is so mystifying is the similarity between everyone's memories of the perfect picnic. So just when did we experience that magical meal spread out on a blanket in a buttercup-strewn meadow? And how come we can remember every last detail of it when we know very well it never actually happened? You know the one I mean, the one where the guys all wore cricket whites and the women all had flowery dresses that button up the front.

The reasons not to picnic are outnumbered only by the several very good reasons why we should. For every hornets' nest and forgotten corkscrew there are twice as many delectable salamis, melting cheeses and sweet, ripe tomatoes for us to put in our basket. Each beauty spot littered with abandoned fridges and used condoms is easily outnumbered by the thought of loaves of crusty bread, bunches of bushy watercress, fat radishes and slices of cool melon. In other words, it is the food that saves the day. Get that right, and nothing short of a tidal wave can dampen our enthusiasm.

If it is to have a hope in hell of being a success, a picnic must be impromptu. So, rule number one is never set a date weeks in advance. Picnics are part of life in countries with a steady climate. Here, they tend to depend totally on what the sky is doing on the day. We need to get an inkling of what the weather is doing before we commit. Which in this country means doing nothing until the last minute - like we are trying to trick the weather into being sunny.

Then we must find that secret spot. Location isn't everything. We can set out our cloth in a layby if we must. What we have been led to believe that is essential to our perfect picnic is an English meadow, preferably with knee-high buttercups and a patch of clover, a stream tumbling over pebbles and a shady tree under which to break our bread. Yeah, in our dreams.

With one half of the British countryside under a blanket of yellow rapeseed and the other half turned into the dreaded golf course with its fair-isle clad clones we can no longer be quite so fussy. Hardened picnickers know that anywhere big enough to unpack a basket is fair game. Maybe our dreams have led us to be a little too fussy about the location. I once encountered an entire Parisian family, including grandmère and several aged aunts tucking into a cold chicken and palm-heart salad on the edge of the Bois de Boulogne. They appeared quite unfazed that less than 10 feet away in the bushes a short, balding gentleman was being serviced by a six foot Brazilian hooker in a blonde wig.

Simple is, as always, best. Fancy food looks silly once you get it out on the grass. It is all too easy to look like you have tried too hard. The only exception is at a black tie picnic, perhaps with Vivaldi (look, if I'm going to be corny then there's no half measures) playing in the background. OK, so I might feel a right twit in a white jacket with a glass of pink champagne in my hand, but not everyone does. Even so, frilly food has a habit of curling up and cursing your guests, and anyway, nothing looks worse than a picnic where you have plainly gone to too much trouble.

Keep it simple. Imagine, then, taking a joint of clove-studded ham out of your wicker basket and slicing off some paper-thin pieces with a long, thin carving knife. You lay them on a plate, together with a handful of rocket leaves (they travel better than fragile lettuce) a cluster of tomatoes still on their vine and a dollop of herb-flecked potato salad. Now pass round the French mustard that you have remembered to pack. Suddenly everything looks quietly accomplished, understated, cool. I have to admit to having a soft spot for rained-out picnics. Not out of a sense of spite but because I like the smell of sandwiches, wet dog and flasks of coffee in a damp car. Nostalgia no doubt, but it is as fine a seasoning for a meal as salt and pepper. As far as this picnicker is concerned, five people in a car passing round greaseproof paper packets of smoked salmon and chunks of pork pie is far more fun than pushing a china plate of oeuf en gelée around a white linen picnic cloth.

I don't think we should concern ourselves too much with bells and whistles, we just need to get the basics right. By which I mean cold, cold beers, enough bread, lemons and napkins (a plastic Jiffy lemon and a packet of Wet Ones just isn't the same). The food must be the sort that travels well, which means leaving melons, roast chickens, loaves of bread et cetera whole and carving them on site. Anything cut beforehand will only dry up. And, talking of little things, we need to get something straight. There must, absolutely must, be proper glasses. Nothing takes the edge of picnic like drinking wine, or worse, champagne, out of paper cups. A cold beer may taste better out of a bottle than a glass but only a drunken hooray would swig bubbly out of a bottle.

Be generous. No one will thank you for that mean little tub of hummus and packet of pitta bread you bought at the corner shop. There must be plenty. A whole cheese costs less than lots of tiddly bits but looks vastly more interesting. A big bowl of cherries will make 10 times the impression of a chopped up fruit salad. Think big, think simple.

It is worth remembering that no food likes to travel. While we are pootling round the country lanes desperately trying to find a green space without the ubiquitous abandoned mattress, those perfectly ripe avocados in the boot are slowly being rocked into guacamole. My experience is to roll up everything you can in clean tea-towels. They protect even the juiciest nectarine, keep cool things cool and cushion breakages. They can also act as outsize napkins for people like me who find eating cross-legged on the floor means almost as much food goes down their shirt as goes in their mouth.

If all this sounds a bit like battle plans then that is only because that is what we will need. We can organise the food, the location and the friends, we can pack our picnic with care and even remember the mustard, but we need to remember also that we are arguing with something bigger than us all. The simple fact that the British were never born to eat outdoors.

Things that always go down well:

Pistachios in their shells. Take a big bagful, they will disappear in no time, especially if you have brought bottles of beer along too.

Sticks of celery - take whole heads that you have washed as best you can. They stay crisper that way.

Bunches of radishes - get them really cold before you pack them up so that they are very crunchy. Don't forget to pack some salt for them.

Olives - make sure they have stones in otherwise they will be eaten in seconds.

Grilled chicken wings to nibble - brush them with olive oil, dust with paprika and cumin and grill till the skin is deep gold and a bit black at the tips. While they are warm, dress them with olive oil, lemon and chopped mint.

Turkish delight - it doesn't melt like chocolate.

A big tub of Smarties - they don't melt either.

Watermelons - make certain they are really juicy and that you bring a big knife to cut them up.

Halved lemons (everything tastes better with a squeeze of lemon over it).

Cubes of nougat, individually wrapped, would be very welcome.

Champagne, or at the very least sparkling cider. Nothing is quite so enjoyable as drinking champagne in the rain.

· To order Nigel Slater's new book Thirst (rrp £12.99) for £10.99 plus p&p, call the Observer book service on 0870 066 7989.