Guys:
Can’t go wrong with a flannel, but again this depends on where you are hiking. Wear layers if you will be in a low temperature zone or going high into the mountains. I personally like to dress nice for most of my hikes wearing something rugged that looks good, REI, Columbia and North Face have great options for outdoor clothes. In the summer be sure to keep those boots on and not switch out for running shoes to avoid injury.
Girls:
Obviously I’m a guy so my advise on what girls should wear may not mean anything to you but here are some things I would recommend. North Face boots and jeans with a thick green jacket is a good look. REI, and Columbia also have really nice outerwear. Something warm is key for the winter but in the summer you can definitely dress lighter but don’t switch out your hiking boots for running shoes, there is a huge difference, and boots are going to be much more beneficial.