He kept his clothes plain, simple, well-fitting, well-cut and, no doubt, expensive. But that's not enough to acquire style icon status. Kennedy went further - he looked comfortable with what he wore, wearing even formal clothes with casual confidence and elegance.
It can be hard to do this. Try to feel at home in your clothes. Unless you're an extrovert, happy to wear high fashion, bright colours, unusual styles, stick with simple classics - particularly as an older man. Try to carry yourself in a relaxed manner - too stiff and your clothes won't sit well and the effect is ruined.
If you find it hard to relax when wearing a suit, try to imagine you're wearing your favourite jeans and casual top and act accordingly.