Terry Pratchett couldn't invent a more complex play on the space-time continuum than the fashion calendar. In theory, at least, it's simple: several months before designer collections hit the shelves, the catwalk shows give a pretty clear glimpse into our sartorial future, followed by a handy period of preparation. However, it gets a bit more complicated when you factor in pre-fall (autumn) collections, which hit the shops around May and June, cruise collections, which start landing in October and November, and the fact that winter coats go on sale at the hottest time of the year, in high summer. Not to mention trying to look au fait with the season we're actually in.

What might seem like a simple question of, "Should I buy that Zara ruffled dress because it's very Stella McCartney even though ruffles might be as cutting-edge as a fax machine by September..." actually taps into deeper issues. It's carpe diem versus forward planning: how are we to live in the fashion moment when we already know what's around the corner?

Of course there are some unpredictable variables. If a reality-TV reject is photographed in a dress, it will be on a fast track to the bargain bin. Conversely, Alexa Chung might be spotted in a formerly passé garment – say, bootcut trousers – only for them to be catapulted to the top of our wishlists. Some trends take off more than others, but otherwise our fashion fate is practically sealed.

The answer to negotiating this existential conundrum is compromise. The perfect wardrobe takes meticulous planning, but who wants to apply the same calculation to choosing clothes as they do a mortgage? Sometimes a cavalier streak helps us break out of a rut; many of my better purchases have been made after spontaneously swinging by Selfridges or Topshop on the way home from drinks in town.

So here's how the perfect spring/ summer shopping plan might look, with a few scheduled windows for living in the moment: note the mood at the autumn/ winter 2010 collections, where grown-up, covered-up clothes and sophisticated practicality were on the agenda, but don't let that knowledge rain too heavily on your spring/summer parade. Buy a few key pieces now, perhaps a khaki parka (APC or Gap), some heeled desert boots or similar safari-ish footwear (River Island or Kurt Geiger), a drapey abstract-floral dress (Whistles) and a blush pink blazer (Zara or Jaeger). Then pencil in some spontaneous buys for say, May, when more maverick, micro-trends appear. The best laid plans and all that...