3)
For males, belly bands can work very well. Since this breed is very tiny, one like
Wegreeco's Male Dog Adjustable Wraps in Size Extra Small
is a good choice.
These not only soak up any pee should the dog urinate, but also just wearing one can stop marking since most dogs do not enjoy the feeling of a soaked band. They may cease the behavior to avoid the aftermath.
#6 React to marking attempts.
In our previous point (#5), we mention the playpen; if you do not have your Chihuahua in his pen, this means that you are able to keep him right beside you and you are ready to react to any attempts to mark.
Any time that your Chihuahua makes a motion to spray, make a loud noise to interrupt this. Some Chihuahuas will respond to a loud clap. For those that do not, a behavior interrupter device like
The Company of Animals Pet Corrector
often does the trick.
As soon as the noise has caused your dog to take pause, bring him outside to his designate bathroom area (even if you must carry them there). If your Chi finishes peeing outside, give enthusiastic praise and a small training treat for reward.
#7 Clean urine marked areas properly.
It might seem natural to use hot water and soap or some sort of all-purpose spray to clean up areas that a dog sprayed his urine, but doing this will not help at all.
Urine contains very strong enzymes that dish soap and other cleaners do not remove. When these enzymes are left behind, this will cause a lingering odor that you will not be able to smell, but your Chihuahua will. It is essentially like having a huge sign that announces 'This is the bathroom area, use it again!'.
So, you will want to wash any spots (old and new) with a cleanser designed to eliminate enzymes that are found in urine.
To Summarize:
Once you are certain your Chi is indeed marking and possible health issues have been ruled out, consider having your Chi spayed or neutered, establish yourself as leader, resolve any hierarchy disputes in multi-pet households, limit outdoor views (if applicable), limit your dog's ability to mark, properly react to attempts, and use a quality enzyme spray to clean urine in the house.