some rules.
wear all one colour with smart jewellery.
NEVER more than three colours
NEVER wear a pattern
wear v-neck sweaters, t shirts and strappy tops (they make your neck look long i.e. you look slimmer and more elegant)
A tailored jacket will make practically anything look chic. (make sure what you wear underneath is all one colour)
black jeans and sweater or long sleeved t shirt (v neck or low neck see above!) with a jacket, long cardi or waistcoat will look effortless.
wear at least one tight(close-fitting) item but never more. - Never wear everything tight or everything shapeless.
Designer handbags are worthless, (unless in-laws are so airheaded that they will judge you by handbag in which case definately flash one about)
Will try to think of some more.