My mum always told me to buy good underwear and good shoes. Cheap versions of both can make the most expensive clothing look terrible whilst good versions will make the cheapest outfit look good. In the UK it is recommended you get measured every six months and that you start off with first hooks on the back, after a while you move up the hooks as the fabric stretches, by the time you get to the tightest hook it's time for a new bra as the fabric has stretched/washed out.