Taking the time to compose yourself as much as you compose your outfit, hairstyle or make-up can actually make you feel more powerful than any sharp suit will. So how do you tap into that confidence? If you're not naturally confident, it make take some practice, but "acting as if" can make a huge difference. Try thinking about someone who presents a confident face to the world. Put yourself inside their head, or better still, put them in your head. How would they stand? What would they say? How would they act. It could make all the difference.
Have you ever noticed that if you wear "that" shirt, or "those shoes", or "the dress" that you suddenly feel more confident?
It's not really the clothes which change your mental state, but your attitude when you wear those clothes. We all have that tried and tested outfit we wear when there's a big presentation to make at work. No-one wants to be distracted by worrying "does my belly look big in this" when they're trying to impress the boss.
By the same token, you can look good in any outfit if you feel confident when you wear it.