On the checklist below you'll find some great suggestions of things to consider packing. Your destination and length of trip will determine which items you need and quantities to bring.
Clothing: For trips of 5 days or less, I bring enough clothing to last me the whole time. For longer trips, I assume that I'll do laundry and actually pack fewer clothes. For 5 day trips, I bring 5 shirts, 3 pairs of pants and 6 pairs of socks and underwear (in case I get caught in the rain, I'll have a clean dry set). For longer trips, I usually bring 3 shirts and 2 pairs of pants (Since I'll be doing laundry along the way, remember I have one extra of each in the clothes I traveled in). In both cases, I try to be sure I can mix and match the shirts and pants to create several different outfits. If a tie and/or jacket will be needed, I bring neutral colors that can be worn with several different outfits. In case of lost or misdirected luggage, I usually carry 1 of my shirts and a set of underwear and socks in my carry-on.
Shoes: I generally wear a pair of comfortable shoes for traveling; preferably slip ons for ease going through security. I will often pack a pair of tennis or walking shoes and a pair of dress shoes. If space is a concern, I'll wear the tennis/walking shoes and forego the slip ons. I have a couple of pairs of dress shoes that work fine with business attire, but can also be worn for a more casual evening out.
Outerwear, Swimsuits & Pajamas: The expected weather at my destination will determine whether I bring a jacket, a coat, a sweater or anything warm at all. Often I make sure that one of my shirts is long sleeve and bring a light jacket. Since I always have a t-shirt, that can give me three layers of extra warmth if needed. If space is a concern, you can usually carry your jacket and find room in and overhead bin, under the seat in front of you or even hung up by a flight attendant. Some planes can get chilly as well. I'm not a big swimmer, but I usually throw a pair of trunks in my bag if I'll have some downtime. They don't take a lot of space. I also don't wear pajamas, but a pair of sweat pants and t-shirt can do the job and double as workout/leisure wear.
Rain Gear: I normally pack a cheap foldable umbrella and poncho.
Money belt & Money: A money belt can often be a good idea. See my 15 Tourist Safety Tips for the Southeast article. Your money supply should be a mixture of debit cards, credit cards, cash and perhaps some traveler's checks.
Documents and Photocopies: Be sure to make copies of all your travel documents and keep them in several places, including leaving a set with someone you can trust. If your destination is outside the country, you'll need your Passport. You should also have your Driver’s license or ID card, and again, if you are outside of your home country, possibly an International Drivers License. Remember your Student ID, AAA card or other specialized form of ID as well as insurance information. Don't forget copies of your e-tickets as well as hotel and car reservations.
Toiletries and Health: This is really up to you. I recommend the bare minimum. Most hotels provide at least soap and shampoo, but if you're staying at a hostel, you may need to bring both. (Speaking of hostels, you may also need to provide your own towel and even linen.) You should definitely bring any medication you need, in the original container, along with a copy of the prescription. A first aid kit can be a good idea, or at least a few bandages, some alcohol prep pads and some pain killer. Don't forget glasses/contacts and a copy of your prescription.
Electronics: Again, this is up to you. I always have my Kindle Fire with me. That way I have several books to choose from. I can watch movies, listen to music, check and send email, play games and even do some work. It even works as an alarm clock. If sightseeing is in your plans, you may want to bring a camera, though if you have a cell phone, it may double as a camera. Whatever you bring, don't forget the charger and, if necessary, a power adapter.
___ Shirts/Blouses: long- and short-sleeve
___ Sweater or Lightweight Fleece
___ Pairs Pants/Skirts
___ Pairs Shorts
___ Underwear and Socks
___ Pairs Shoes
___ Rainproof Jacket with Hood or Poncho
___ Tie or Scarf
___ Money Belt
___ Money—your mix of:
________Debit card (for ATM withdrawals)
________Hard cash (in easy-to-exchange $20 bills)
___ Documents plus photocopies:
________Printout of airline eticket
________Student ID and hostel card
________Railpass/car rental voucher
___ Electronics—your choice of:
________Camera (and related gear)
________Mobile phone or smartphone
________iPod (or other MP3 player)
___ Chargers for each of the above
___ Empty water bottle
___ Wristwatch and alarm clock
___ Earplugs/neck pillow
___ Toiletries kit
___ Toiletries (soap, shampoo, toothbrush, toothpaste, floss, deodorant, sunscreen)
___ First-aid kit
___ Hand sanitizer
___ Glasses/contacts/sunglasses (with prescriptions)
___ Sealable plastic baggies
___ Laundry soap and clothesline
___ Small towel
___ Sewing kit
___ Travel information (guidebooks and maps)
___ Address list (for sending postcards)
___ Postcards and photos from home
___ Notepad/journal and pen