Accessories! I definitely think it's possible to take the same jeans and t-shirt look and make it look as funky/edgy/punk/cutesy/whatever as you want. For instance, if you took those jeans and t-shirt and added a j. crew cardigan, loafer-ish flats, pearl earrings, and a wide fabric headband, you're going to come out looking kind of preppy. If you take the jeans and t-shirt and added a leather/pleather jacket (like this: http://store.delias.com/item.d.....andFilter=), some long chainy necklaces ( http://www.wetseal.com/catalog.....ctId=16590 ), a bunch of mix-matched bangles ( http://www.forever21.com/produ.....38;Page=1# with http://www.forever21.com/produ.....038;Page=2 maybe) or a big cuff (even something like http://www.urbanoutfitters.com.....CCESSORIES), and some cool shoes. Throw on a bit of dramatic makeup and you're good to go.
I think the key is figuring out which elements of the punk look appeal to you, and then finding them in more sophisticated pieces. If it's wearing a lot of black, well, you can buy black clothes anywhere. If it's the metal influence of pyramid belts and zippers and things, look for belts/jewelry with chunky metal features. If that's what you do like, you're in luck because zippers and pyramid studs are everywhere these days. For myself, I know that even if I'm wearing the girliest of outfits, I always feel a little more 'tough' because of the way I usually wear makeup (rim my eyes all the way around in black eyeliner, with bright green eyeshadow and a bit of blackest black mascara, which you can sort of see here: http://img.photobucket.com/alb.....hoto13.jpg).
That was long, and with a million links. Hope it's at least a little bit helpful!