Just show that you take a pride in your appearance. If you wear a white shirt, make sure that it is white and ironed. A smart jacket and well pressed trousers will go down well. Shoes should be polished. If you are not used to wearing a tie, don't bother; you will inevitably look like you don't know how to tie one properly. Above all else, be yourself.
For my interview I wore jeans and did loads of sit ups and press ups in the recruiting office. I understand from Ninja that were a recruit today built like me when I joined up I would not be allowed onto the PRMC. I was from a kids home, but also a county cross-country runner (middle of the field) and a county gymnast (skinny arms but very strong on floor, horse, beams and especially rope climbing). I have said this before, when I got to Deal they nearly sent me back to the Home because they could not fit all the pouches onto my G1098 belt. My mag pouches were above my thighs pointing forwards. After an initial run and beasting I specifically remember the training team saying something along the lines of 'We're training commandos not ******* guards so give the ****** a chance'. And so I was in, my 'adult' kissed me and gave me a hug and signed me off! (I was so embarrassed). I was not yet 17