I have traveled extensively with two teenaged girls, including two winter trips to Prague and Vienna and this is our packing strategy:

Each family member brings one 22" rolling suitcase , one empty small/medium duffle bag folded flat in the suitcase, and a backpack (bookbag size) for carry on. The things we purchase must fit in the duffle (fragile things in the carry on). The backpack frees up the hands to deal with the other two pieces of luggage, and we usually clip the duffle onto the rolling suitcase. We have found great deals on suitcases at Marshall's.

The coat layer is worn onto the plane. As I hate to do laundry on my vacation, we bring enough underwear for the whole trip (doesn't take up much space, although I know people that follow the wear and throw away plan with underwear). One "nice" outfit, could be slacks. Two other pairs of jeans/pants. Two sweaters or fleece, several shirts that mix and match. Lightweight t-shirts can be worn under other tops to keep them fresher. Some cute accessories and scarves. No more than 3 pair shoes, 2 is better (wear one).

We try to get our shopping day/half day out of the way early. My girls love Promod, Zara and H&M, none of which we have at home. They always want to wear the new clothes on the trip, which helps stretch what they have brought.

We have found this really works even for bulky winter clothes. Spring and summer are even easier. A big help is to roll up the clothes in the suitcase. The first time I tried this was in desperation returning from a 3 week trip, with an impossible amount of new purchases. I couldn't believe how much difference it made!

We took our 2 teenage girls to Paris & London last year, so we had a little more space. They are the same size, so they basically had 1/2 full suitcases and saved their money to buy lots of clothes. A fleece and jacket (like North Face) are a must and can be worn together or separate. Jeans were very prevalent in Europe. A nice purse is also a must as first impressions are important. Be mindful of the suitcase weight as airlines are charging for overweight suitcases. Be sure to check out Sephora for cosmetics, your daughter will love it. My younger daughter has a size 9 1/2 foot and had a little trouble buying shoes. We got department store discount coupons from our hotel, and Printemps and Galleries Lafayette both have junior departments. A good tweed jacket/blazer served for both day and evening. Warn your daughter that European men like to acknowledge pretty girls - my daughters said they were pinched several times in Paris. It wasn't threatening, but men flirt a lot more openly than in the US. Each girl used their giant school backpacks as carryons to contain books, cosmetics, etc. You can get about 50-60 lbs. in there. Ditto on rolling up the clothes, especially underwear, tops, etc. Try to encourage no more than 2 pair of shoes, one daughter was desolate that she couldn't fit her Ugghs in the suitcase! Have fun!

she could have worn the UGGHs on the plane
;-)

Along with her velour track suit...:)

i still think each person having their own suitcase AND having to carry it themselves, is the smoothest way to solve the problem. I like Noe's method best.