We have (in reality, Mrs. C has) a Lhaso/Bichon cross (euphemistically called a Zuchon, here in the U.S.) and the second most difficult training item was house breaking (the first being getting her to come when called). Mrs. C. followed the example already given of taking the dog outside every 30 minutes. If she was fortunate enough to get the dog to do her 'business' she (Mrs. C) always returned exactly to that spot when taking her back out.
I notice, when walking the dog, (she's now about 3.2 years old) that for a breed thought to be a 'house' dog she investigates every thing by smell. Suppose it's the spaniel influence in the DNA... but this is helpful in house breaking since she will urinate at every site previously visited by another dog.
The saving grace was that the little dog now associates going outside with excitement, which, as you've discovered, seems to be the whole raison d'être for her... since she is the center of the universe...
Best of luck...