Savvy travelers know how to pack because of their experience in my opinion.  I am a perfect example for packing light now because during my first trips I packed everything and the kitchen sink.  As I gained experience and learned that carrying too much luggage is a hassle especially if you take trips to different cities and use trains and or buses.  After every trip I evaluate what I brought and if I used anything less than 2 times then I really did not need it.  In most cases if you need more items you can purchase it at your destination and it is fun to shop in foreign locations.  Last year during our trip to Paris France the weather was cold and rainy so we had to buy warmer clothes which I wife did not mind at all!  We only packed 1 carry-on for a 10 day trip to Paris.  The trick to packing light is all about experience and as you travel more you will enjoy your travel experience much more if you pack light.
http://www.bbc.com/travel/blog/20130701-packing-secrets-from-savvy-travellers