Below are a few things we do:
1 - On our registration sheet (with our rules etc) which we ask all guests to read and sign, we have a section saying, "I also understand that The Emperor's Crown is not responsible for the damage or loss of any personal belongings."
2 - Every bed has a locker big enough for a suitcase, which can be locked with a standard padlock. We don't provide the padlocks although we do sell them in our vending machine. Making the guest use a padlock that belongs to them takes away some of the paranoia, whereby a guest could think that we have spare keys cut for padlocks we rent out or something like that.
3 - Nobody who is not staying in a particular room is allowed to be in there. Obviously this cannot be enforced at all times but it does help prevent guests bringing randoms in as we've found that this is when thefts usually occur...
4 - We have both a short-term and long-term luggage room which is free of charge (only if they have a future booking for the long term room). We make guests aware that the storage room is safe, however we don't take any responsibility for items stored there.
5 - We have CCTV covering all our corridors and entrances which I skim through daily. If you don't have CCTV in your place, get it. You would be amazed at how much easier it makes life.
6 - Our cleaning staff are outsourced through a professional cleaning company who vet their staff pretty well.
7 - Guest screening. Possibly the most important thing. I find that you can tell who the troublemakers are as soon as they walk in. All my staff know to refuse beds to anyone they even get the slightest bad feeling about. I'd rather lose some revenue than gain some problems.
8 - Reputation plays a big part. We take a hard line on any trouble and so we seem to get less trouble. Maybe word gets around?
9 - ID from every guest. Passport or Australian Driver's License, no exceptions. People are a lot less reluctant to steal when they know you have their details if they are caught.
10 - Guests can store small personal items in our safe, however I stress to them that I do not take responsibility for the items.
In all honesty, no matter what you do, stuff will be taken, usually when a guest doesn't take the right precautions (lockers are there for a reason).
With regards to avoiding responsibility for theft, I think as long as you get the guests to sign something which states that you don't take responsibility for their belongings, you should be fine.
We haven't had any theft in over a year from our rooms. However, our fridges are a different story. Food gets stolen pretty regularly and with the way our kitchen is laid out I don't see a way to stop it. It's very hard to spot on CCTV if someone is dipping into the wrong bag in the fridge.
I would love some custom made fridges with separate, lockable, compartments for each bed/room!