1) Get everything you want to pack in a pile. Put half of the stuff back in the cupboard.
2) Don't pack trekking boots. Unless you're planning on serious climbing you will not need them.
3) Leave the sleeping bag at home (unless you're planning on camping).
4) Pack one book and one only. There's loads of bookstores and bookswaps across the region. Read and swap -- carry less.
5) The biggest mistake of all - don't leave with a full pack!
Planning well is an integral part of getting the most out of your trip. Be it picking the right backpack, the right vaccinations or the right country, the simple decisions are often the most important.