1) Buy only if 50 % off or more. 50 % is perfect. I gives you actual value for money while the designer still gets rewarded for their hard work and imagination.
2) Buy only clothes which are in season. Worst mistake is buying a big ticket item like a coat out of season. When winter finally hits you will be totally sick of it and you'll be tempted to buy a newer fresher version. Instead, buy bikinis in the summer sale and coats in the winter sale. Lots of swim wear goes on sale in June (I just bought some) and coats go on sale in November. So plenty of time to wear them for the seasons they were intended for.
3) Buy only things you already wanted before they went on sale. Put everything you would love to have in your online shopping baskets. If it's too expensive for your budget, wait till it goes on sale. If you miss out, do not get the next best thing just because it is on sale.
Lanvin bracelet, Pucci bikini and Miu Miu one-piece - my only sale purchases this season - nothing else passed the rules