While some dogs that are fixed will still mark, spaying or neutering drastically cuts down or eliminates these issues for most dogs when in conjunction with good potty training rules.
2. Establish yourself as leader.
Most owners assume that their dog sees them as the leader. However, this is not always so.
In many cases, a dog will mark in an attempt to show dominance, either to challenge their human's place in the hierarchy of the house, or to see if there is any weakness. It's the canine way, even for small toy breeds like the Shih Tzu.
In addition, a common issue seen is that a Shih Tzu will pee on his owner's bed, shoes, or other personal belongings. This can seem like a personal insult. However, it is often just the opposite.
A dog, believing he is Alpha, may urinate on his human's belongings or right near them, to mark the area in an attempt to ward off potential dangers to his 'Beta' (which means you, if the roles have been reversed).
You can claim your role as a loving leader by only placing meals down after a 'Sit' is obeyed. In addition, whenever leaving or coming back into the house, you should be the first to do so, followed by your Shih Tzu. The house is the 'den' and the leader always is given the respect to enter and exit first.
In some cases, a dog will mark in response to other animals. This may be a neighbor's dog or even wildlife in the yard.