1. way more socks than you think you need. if you are going to be walking any distance make sure you know what works for you. Many hikers like to wear a pair of sock-liners under a pair of wool socks even in summer in order to ruduce friction. Mole skin can help with trouble spots. Cotton socks are disliked by most hikers, but know what works for you one very old hiker who hiked the alpalachian trail recently doesn't use socks at all he just carries around a lot of baby powder, not something I would recomend but to each their own. Make sure you are familiar with your boots or shoes before taking any serious trips. Wear stuff in and get comfy with it before taking equipment on a serious trip not only shoes but other stuff as well.
2. don't forget the underwear
3. pants and shorts if appropriate. Jeans are tough and provide good protection against brush and scratches, but take up lots of space in a pack and dry very slowly, and are not very warm. A pair of khakis or cargo pants pack in a smaller space and dry faster but also aren't very warm. In the winter be sure to pack some long johns, cotton ones are ok, but a pair of polyproplyne long johns help wick moisture away. If it's really going to be cold bring along some insulated coveralls or a pair of insulated or wool pants. In warmer seasons bring along a pair of jeans along with your shorts just in case you need some protection for your legs.
4. In cold weather your torso is the last place you want to skimp, that's where all that important stuff is like you heart and lungs. There both pretty picky about the temperature. A nice warm vest will help keep your toes warm belive it or not. Dress in layers, this is for two reasons. First, it's warmer. The air that get's trapped in between the layers acts as an insulator, and second it's easy to shed layers if you get to hot or put layers on if you get too cold. In sunny weather you may want to consider lose fitting light and light colored long sleeved shirts vs. short sleeved t-shirts. this will help prevent sunburn.
5. gloves and mittens. Ok, I know mittens are dorky but they are far warmer than gloves. I am not a fan of thick bulky gloves they limit the use of your fingers almost as much as mittens do and they are cold. Know once again I'm going to push layers. a pair of thin "glove liners" to go inside mittens will allow the use of your fingers without exposing your bare skin, sometiems this can be verry important. when chosing mittens and gloves keep this in mind. Cotton bad. wool is nice but annoying if you are going to be playing in the snow. Snow sticks to wool, wool that is wet may be of some instulating value but snow packed wool will keep your fingers at a toasty 32 degrees. The typical nylon coated usually "thinsulate" type mittens and gloves bring the best of both worlds to you. Often glove liners inside the mittens seperate your fingers reducing their collective warmth so if your fingers are cold you may want to forgo the liners but if this is the case you probably need to bundle up more in general cold fingers and toes, if properly insulated usually point a problem of losing too much of yoru body heat all over. 5. Necks are something that often get neglected. don't forget the scarf and pay attention to collars when buying coats, especially if you plan on getting into many snowball fights. in the summer don't forget the suncreen necks tend to burn easily. 6. Hat's are very important in the summer and the winter. You lose over 80% of your body heat through your head. don't forget to cover it up in winter. knit stockigng caps work very well, coverd up with a hood they work even better. Earmuffs that just cover up ears and not the head are asking to be cold. In summer hat's are also very important, a light colored wide brimmed hat, prefereably one that goes all the way around to protect your eyes and the back of your neck. One you don't mind dunking in the water and putting on your head can make hot days more tolerable.
Do not sleep in the clothes that you wore that day, either bring some pj's or change before you go to bed. Even on the coldest days you sweat and that moisture gets trapped in your clothes. if you sleep in them you will be cold unless it's hot out in which case you will just be sticky sweaty itchy and miserable all night. And remember wool keeps warm evern when wet, cotton sucks but it's usually ok in summer and besides it's cheap and comfy. high tech synthetic fabrics like polyproplyne and gore-tex, and those spiffy fills they use instead of down in coats and sleeping bags usually work well but can be expensive. Down whether in a coat or in a sleeping bag is the warmest material by weight around and it packs in a very small space, it's drawbacks include the fact that once it gets wet you are screwed. Down does not keep warm when wet, it takes forever to dry, and it's expensive.