Get a sturdy bag - preferably a backpack or convertible travel pack - that you can wear or carry comfortably. Remember that you may have to walk or take public transit to your hostel.
Pack slightly less than you think you'll need - you can always do laundry if necessary. Leave behind clothes or shoes that you aren't sure you will wear.
Bring along a few tiny, lightweight souvenirs to give to people you meet on your travels. These could be postcards from your hometown or tiny lapel pins with your country or state flag.
Bring clothes that don't need ironing or dry cleaning. Choose items that don't wrinkle easily or show stains. (Leave the white linen shorts at home.)
Make sure every item of clothing you pack can be worn with at least two other items. This step will allow you to mix and match outfits without packing too much.
Roll your clothes instead of folding them. This step will mean fewer creases and more usable space. Tuck small items inside your shoes.
Put liquid toiletries, such as bottled shampoo or perfume, inside a zip-lock bag, just in case a container splits or cracks. In general, it's a good idea to decant toiletries into small containers, depending on the length of your trip and your destination. (Remember that you can buy the basics just about everywhere.)
Pack a small address book, so that you can exchange contact information with any new friends you make while hosteling.
Bring a few miscellaneous essentials: a needle and thread, a set of utensils (plastic or metal), a few safety pins, a length of string or cord (for an impromptu clothesline or to tie things to your backpack) and some extra zip-lock bags. Pack a travel-sized container of sanitizing soap, if you have room.
