One of the things that bugs me a bit about dressing 'advice' for grown-up women is that it assumes you've let yourself go and your main objective should now be to disguise your enormous pot belly and disgusting wobbly thighs.

But actually, being a bit of a fitness fascist, I haven't got those things - in fact I'm slimmer than I was at 18. Nevertheless, I'm aware that this is a minority state of affairs - most women gain a good 20 pounds in weight between the ages of 20 and 45 and retain weight until their 60s, at which point they start to lose it again, so weight is a major issue for many women.

Nevertheless, dressing well shouldn't be simply a matter of weight, and it shouldn't simply be a matter of age either. We're all aware that a 40 or 50-year old can't dress like a teenager - that's just plain sad, but once you hit 30, I reckon, you can start developing a personal style that can take you, with annual updates, through the rest of your life. So what should it be based on? Here are some handy 'rules' - rules in the sense of 'for the obedience of fools and the guidance of wise women'...

1 Dress like a grown-up

You are not a little girl any more, so knock off the ruffles and bows and all the cutesy, ditsy stuff. Shorty ra-ra skirts, itsy-bitsy little tops, girly prints, t-shirts with bunnies on. These are no longer for you. Instead, look for reserved, adult clothing with some structure and shape to it. Long sleeves on tops, long legs on trousers, whatever necklines are most flattering for you personally, clothing without bells and whistles, classic block colours - black, white, navy, cream and good neutrals. Build your wardrobe around these items and add your own twist and flair.

2 Keep it covered

I don't mean nun-like, but in general, unless you're in Madonna-like shape, follow the 30 per cent rule - in other words, only show 30 per cent of your body at any one time, even for evening. Now is the time to look like a woman who's actually getting sex, rather than desperately looking for it. If you've got great arms, by all means wear a sleeveless top, but keep the neck high for maximum impact and cover your legs. If you're wearing backless, keep the front high: if you're wearing a plunge front, keep the back high... The fact is: if you reveal your flesh, you are going to be compared with every 20-year-old who does the same - it is far better to leave people guessing about how gorgeous you are than to show them you're not.

3 Keep it simple

There's a good reason I've banged on endlessly in this blog about 'classic clothes', and that's because they work. And one key thing that differentiates classic clothes is that they have simple lines - their design is pared down to the essentials.

Whatever you're wearing, seek simplicity and avoid exaggeration. Don't wear things with 25 colours and added bits of gewgaws all over them - contrasting appliques and heavy beading in clashing colours. Avoid big shoulders, poofy skirts, huge floppy collars and lapels, and weird sleeve designs (currently being foisted on us by the shedload). These don't do anyone any favours - even teenagers, but teenagers have a right to look stupid if they want to. Grown up girls need to raise the bar a little - aiming for elegance and class. Keeping it simple works with any type of clothing - blouses with small collars, t-shirts with scoop or v necks, blouses with clean French cuffs, pencil skirts, clean-lined jackets with vertical seaming....

4 Keep it clean

When I say clean, I do mean physically clean. Being scruffy is the prerogative of the young, the rich and the mad, but the rest of us have to conform a little even if we may not like it. Going out with chipped nail polish, undyed roots or clothes covered in dog hair just screams middle-aged rut, and don't think that people won't notice because they will. Grown-up girls have to look groomed. Not polished necessarily, but soignee, as the French say - cared-for, put-together. A clean, crisp, groomed appearance always works, no matter what your lifestyle.

Yes, it takes a little application, but the effort repays itself a hundred-fold. Choose a haircut that you can maintain easily (or pay to have maintained). Keep your clothes clean (if you can't afford dry cleaning, buy clothes you can wash at home). Do running repairs once a month - sewing buttons back on and taking your shoes for re-heeling. Iron things properly and treat stains before they set. Overall, treat your clothing as if it cost ten times the price.

5 Keep it quality

Quality wears better than rubbish, and whatever the item, quality materials, cut and finish will show. Buy quality items wherever you can, even for basics - pima cotton t-shirts, Egyptian cotton blouses, decent wool-rich suiting (a little stretch here can work wonders), a fantastic pair of jeans. Watch out for the sales and stock up on basics from good manufacturers. It is better to have a smaller wardrobe of quality items than a large wardrobe of tat.

When I say quality, this is quality at every level, so if you're strapped for cash, go for the best of a type. Rather than buying low-end fakes of high-end items, look for high-end democratic items at a lower price level (as an analogy, it's better to eat well at a mid-price restaurant than to toy with your starter and dread the bill somewhere expensive). Instead of tinny gold-plate jewellery, buy handmade wooden beads; instead of a plastic leather-look handbag, buy a good-quality canvas bag; if you can't afford cashmere, buy merino on sale rather than a cheap acrylic sweater.

Tomorrow, five more tips on dressing over 40