For starters, I would say that you need to ensure that your kilt fits properly. Hands-down, whether you are skinny or large, that will speak volumes about how you look. Make certain that you are wearing your kilt neither too long nor too short.

Also, the one other thing that will speak loudly about your appearance is your comportment and how you carry yourself. Walk, stand, talk, and BE confident (without being arrogant) but also not shy or mousy.

Do these two things, and I guarantee you will not look out of place. But I know that's not why you started your thread. So to add a bit of fashion advice, I would also recommend having a proper sporran and kilt belt to complete your look. The belt will help divide your outfit a little into top and bottom so you don't look like one, long noodle, and the sporran will add some substance to your presence as well. Choose a sporran that's of good size -- not too big or small.