You will only want to lug your pack when you’re traveling from city to city. Almost all hostels have someplace where you can lock up your pack so take a padlock. Also, for the most part, your pack contains your dirty laundry which isn’t likely to get stolen. As long as you have all your valuables on you, it’s generally safe to leave your pack during the day even if you can’t lock it up. A bicycle lock chain can come in handy to chain your pack to the luggage rack on the train if you have to leave the pack at the end of the car instead of at your seat. I sometimes carry one and I think I’ve used it once and wished I’d had it with me maybe twice.

I have zipper locks for my day pack and use them occasionally. Locks won’t stop anyone who really wants your pack, so don’t leave anything of value in it. Locks will slow down a snatch and grab attempt so that somebody else’s gear will be a more tempting target. I’ve never had anything stolen although my husband lost a cheap wallet with a small amount of cash to a pickpocket.

There are several folks here who swear by packing cubes. You can find some of their comments if you read old posts in the “packing” forum. I don’t use them. Do take several quart sized zipper bags as they tend to get beat up and you’ll need one for your carry-on liquids on every flight and to avoid shampoo spills in your pack.

I’ve seen those wire bag protectors…they must be something new.

As oldlady pointed out, no one will be interested in stealing your clothes, and just keep your valuables on you or locked in a locker. However, I have been in situations where I would have found one of those wire protectors useful. I always travel with a DSLR, and I have stayed in a couple of hostels that did not provide lockers (nor did I have a private room). It would have been useful to wrap a wire bag protector around my DSLR bag and lock it to my bed, near me, while I’m fast asleep at night. So, it depends on what you travel with. I’d say that getting one of those wire protectors could be worth it. I’m considering buying one myself.