The kennel, physically, should not only be clean, but it should smell clean as well. The bedding the dogs use should also be clean and look "comfy" for your dog. All the sleeping areas should be dry as well as free from drafts. Be sure that your Mastiff has access to an outside exercise area during the day. He should also have access to water at all times. Don't be afraid to ask about the feeding schedule your dog will be on. It should, ideally, stay as close to his home schedule as possible. In addition to this, find out how much the staff knows about their boarders. Not only that, but feel them out about how much they know about dogs in general. While you don't expect them to be experts, the staff should have a love of dogs and some type of knowledge in this area. If you start boarding your Mastiff while he's young - even as young as six months - then he'll generally become acclimated to the environment there. And it's best to use the same kennel every time. Not only will the staff get to know your dog better, your Mastiff won't feel as if the employees are "strangers". He'll feel less like he has to "guard" his territory constantly. One final note: consider a "trial" boarding. Have your Mastiff spend a day or so at the kennel when you're not out of town. That way if it turns out not to work, you can pick him out. Then again, the chances are that the arrangement will work. So that when you do go out of town, you won't have to worry needlessly. You know your mastiff needs extra attention. That's just the way the breed is. And it's partially why you love him so. Don't hesitate to put extra time and energy into your search for a kennel that knows the special character traits of the Mastiff and is willing to give him the time and energy he needs.