Entertainment on grass means going back, oh, about 100 years. It's hard to be modern sitting out in Windsor Great Park (polo) or at Glyndebourne.
In fact it's pointless to even try. Most of the desirable plein-air events are what's left of the Victorian Season, supported by corporate money.
The corporate money thing is key. The green stuff is usually the absolute top end of the corporate spend. They take you away and feed and water you for the best part of a day; it's very well done - and by a very nice class of company too. Pilar Boxford of Cartier is the doyenne of corporate hostesses and she does a lovely line in green entertaining, things like the Cartier polo, garden parties at the Chelsea Physic Garden and Style et Luxe at Goodwood.
A number of these green survivals are royal, some of them explicitly (Ascot), some of them because senior royal people go and do royal things.
What do the sponsors get out of it? Good photo-opportunities, obviously, and a chance to get close to the people they want to know better - big clients, potential clients, key journalists. The fact is that people really want to go to these summer season parties, even those who get asked to everything.
This mix of upper-class turf and corporate purpose makes people behave oddly. At Ascot, according to John Morgan, Britain's answer to Miss Manners: 'People start to feel they're extras in My Fair Lady and they get very Edwardian.'
Anyway, you should be in there, boy; if you can possibly squeeze yourself into Glorious Goodwood or Glyndebourne or Henley or any of the rest on any ticket, you should. Because (a) the spend per head is huge if it's major corporate stuff, (b) it's terrific observation material, and (c) when everyone is thoroughly destabilised by being drunk and playing Edwardian there are all sorts of opportunities to network.
For most people from the sponsoring companies this kind of thing - posh, rich, green, continuous good drink - is a large part of their psychic reward for doing whatever it is they do. It's terribly important to them, because this is often the life they want their nice new money to buy them.
Dress people up for Ascot and they start acting the part. Dress them down/country them up a bit for a race meeting and something transmits itself from the brown shoe leather, the mud-coloured worsted and the dun tweed. There's always a period precedent, a book or a costume drama to slip into. However aggressively nouveau you are, you'll be period nouveau like Tommy Lipton (Edward VII's favourite rich grocer friend, who took him yachting) or an Agatha Christie character or Gatsby.
Go with the flow. A lot of corporate life is acting anyway: absurd structures, ridiculous job titles, a vocabulary learnt from airport books that dates the moment you use it; so a few rituals on grass won't hurt.
The ultimate garden party, however, isn't corporate but institutional.
The royal garden parties are intended as a regular tonic for the nation, as a reward for years of public service, or for a bit of pro bono from straight-arrow profit-maximisers. All human life is there, a regular cavalcade.
You might find your chairman there, basking in his new knighthood, you might find the postroom supervisor, retired five years ago but with a Secret Life raising hundreds of thousands for wheelchairs or sick children.
It's wonderfully levelling and distinctly surreal.
But royal garden parties aren't exactly obvious network opportunities.
That means more modest entertainments. Your divisional director's barbie has its precedents in Australian and American affairs of the '50s, with vicarage tea party and Laura Ashley thrown in.
Anyway, the point is you can't be too archaic for these things - food, manners, dress-code, the lot. Just like black tie, it can be profoundly democratic, simply because the rules are so well documented. For a general guide to the context - ie, who made these green rules and how the calendar runs - I can recommend three wonderfully wise and witty books. The Official Sloane Ranger Handbook by Ann Barr and Peter York sets the tone; The Official Sloane Ranger Diary - same authors - describes the events; and The Official Sloane Ranger Directory is good on caterers and clothes shops.
John Morgan's Debrett's New Guide to Etiquette and Modern Manners or his recent Times Book of Modern Manners will equip the chippiest Essex dot.com made-it-for-life on the Pinewood back lot.