Find out what your airline's baggage constraints are for size and maximum number allowed. Make sure that your luggage meets their specifications.
Choose a large rolling suitcase (enough to hold a semester's worth of stuff) with durable wheels and a sturdy handle for wheeling through airports.
Pick luggage that is highly durable, since it will need to be able to withstand being thrown around a lot by airline baggage handlers.
Make sure that your largest suitcase has a strong strap for attaching another piece of luggage so that you can pull multiple items at once.
Choose a carry-on bag that you can carry across your chest. Lugging around a heavy carry-on bag on your shoulder through international airports can get really uncomfortable.
Layer pants and other bulky items on the bottom of the suitcase and work your way up.
Fold shirts, pajamas and other items and stack on top of the pants. Roll up any wrinkle-prone items.
Use zippered plastic baggies to consolidate socks, underwear, shirts and other clothing items and cut down on the room they take up in the suitcase, making sure to expel all air from the bags. The zippered baggies are especially effective at minimizing space used by sweaters and other bulky items.
Keep toiletries separate in a small bag, putting any leakage-prone bottles in plastic baggies.
Take only enough toiletry-type items, such as toothpaste and shampoo, to last the first part of your trip until you can go to a drug store in the country where you'll be staying. There is no need to take enough of these items to last the entire semester; they would only take up more room in your suitcase, and they can easily be bought just about anywhere.
Pack any appropriate electrical adaptors and a converter. Make sure that any appliances you take can be converted to the local electricity.
Make sure that you take enough contact lenses to last the entire trip if you wear contacts. Because contact solution varieties are not universal, this may be a product that you want to take enough of to last the entire trip.
Take a bottle of wrinkle releaser to use instead of having to iron clothes. Even if there is an iron available where you will ultimately be staying, using wrinkle releaser is much faster and is generally pretty effective.
Include a mesh or plastic bag for dirty laundry to keep clean and dirty clothes separate, especially if the dirty clothes are wet. This will also give you something to carry clothes in when you go to do laundry.
Pack a large duffle bag or other soft, collapsible bag in which to carry home souvenirs and other items you pick up along the way. After spending a semester abroad, you are bound to come back with more than you started out with.