Pare down your packing list as much as possible and don't travel with anything you can't live without. Leave expensive jewelry and heirlooms at home and don't wear anything flashy that could make you an obvious target for thieves.
Find out in advance if the hostels where you will be staying have lockers available. If not, ask what security options they have. Is there a safe at the front desk? A locked room where you can store your bags?
Carry a sturdy combination lock or padlock (wear the key around your neck). You can use it to lock your bags to a bed frame (if the hostel allows this) or another fixed object if necessary, and to lock hostel lockers.
Be friendly, but be careful. Try to keep important personal information to yourself, and trust your instincts if someone makes you feel uncomfortable. Beware of sob stories and requests to borrow money "just for a few hours" - many scam artists rely on the goodwill and gullibility of fellow travelers.
Take your valuables with you whenever you leave the room, even just to use the bathroom or shower (bring a plastic bag along to keep everything dry).
Know where your valuables are at all times, so that you can grab them quickly in the event of a fire or other emergency. In an emergency situation, you will need to get out of the room immediately, leaving your backpack or suitcase behind. You won't be able to waste time hunting for your stuff.
