If you want it to look right you need to go to a professional wig-maker - they will ask for a photograph of you at the age you want to look, see how you dress and take your height, neckline, ears, jaw and the proportions of your face into account. Then they'll make a template of your scalp, so that the wig is a perfect fit.
Of course, not everybody wants a wig to look natural. A lot of my customers are transvestites, in which case looking natural is not vital; what's important is looking female, so all they need is an off-the-peg wig that takes off the hard, masculine jawline. If you want to look younger the colour has to change, but be careful what shade you choose; black and blond are hard colours and show up the lines of the face. I can make a person look 10 to 20 years younger with the right wig, but only if it's accompanied by make-up, so bear in mind that ever afterwards you can't have one without the other or it will just look wrong.
People think of themselves as they see themselves in the mirror, and when they put a wig on that's who they want to see. But that isn't the point; they buy a wig to be changed and to look better. At the end of the day, they have to trust me, because most of the time, people have no idea whether it looks right or not.
The 86-year-old wig-maker Mr Gilbert has more than 3,000 wigs in stock at his shop, Gilbert Hairdressing Academy, 67 Praed Street, London W2 (0171-723 4513) and appears on BBC's `Paddington Green' - the third series is scheduled to start this coming autumn