Fit is King. No excess material, it makes you look bigger than you are. Think streamlined and tidy - high armholes, snug fit on shoulders and torso.
Button Up. A waistcoat is meant to make you look put together, so why have it any other way? Buttons up, gents – but leave the bottom button undone.
Try not to look like a waiter. Go for seasonal fabrics like tweed in winter or cotton in summer and avoid tragic, shiny polyester. Keep it simple – no unnecessary details. And avoid wearing with black trousers.