Keep anything of value on your person at all times. DO NOT let it out of your sight/touch, not even for a couple seconds.
Keep some form of ID on you at all times. (This is a safety tip, as it sucks waking up in the hospital as "John Doe".)
Be considerate of others space, and they will be considerate of yours (in most cases).
Go light on the onions, garlic, and cologne and bathe regularly as best you can (even if it's a cloth rag bath in a bathroom sink).
Bring your own cloth as it's very possible there will be none provided.
Keep an eye on your shoes. (I once had mine stolen and it SUCKED.)
If you are staying in a higher end hostel, some of the advice above may not be necessary.
You may want to take a sheet along, as not all hostels provide bedding.
MOST hostels do not provide toiletries (and some don't have showers).
Headphones are a GOOD thing. Especially if you end up in a room with someone who snores loudly.
Depending on how many people are staying in the room, it could get pretty warm in there overnight. Be prepared for the possibility.
I prefer top beds, BUT... bottom beds are often much easier to settle into as well as get in and out of. If you have a choice, it's something to consider.
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