Taken directly from www.newscientist.com this story caught my attention!
"Auto-snug clothing
Philips hopes that fitting-room fiascos will become a thing of the past if it ever forays into the world of fashion. The consumer electronics giant has come up with a way to change the size, shape and style of clothes by weaving "muscle wires" into the fabric. The wires are made of shape-memory alloys that change length according to the small current passed through them.
Here's the idea: you try on a special pair of Philips' trousers, and connect up to a power source that changes the length of the wires in the fabric until the trousers have the correct waist size, inside leg and width.
Then simply disconnect to try the trousers in exactly your size. Philips says the technique could also be used to correctly fit shirts, socks and bras, or indeed any other article of clothing.
Read the full auto-snug patent application. "
So presumably, if you lost or gained weight, a quick plug into the mains and voila!