What a great topic.
It can be difficult, as I trained 2 Chihuahua/Jack Russell's. Chili is now 2 years 3 months (7lbs) and Harley is 1 year 3 months (4lbs). Chili was house broken in 4 months, Harley was a different story. Being I already had Chili, I couldn't train Harley in the same way. I'd leave Chili out to run around the house all day, while my fiancé and I were at work, but now that Harley is around, we must put them in a crate. When I first brought home Chili, he had to be by himself almost 10 hours, while I was at work. I put him on a wood floor, penned in with a gate. I had his food, water, small crate, potty pad a couple of toys and his blankets, in there. Not one part of the floor was showing. He always pottied on the pad. Once he was about 3 months old, he had learned to hold it all day. When I was there, I'd take him out when he'd get ancy or about every 1-2 hours. Except while I was sleeping, in which, I'd block in him his bed (by a laundry basket) and only if I heard him get up and shake, I'd take him out. I always promptly took him out in the morning and made sure he went #1 & 2 and before bed. He had a few accidents after this. Mostly when my mom would be watching him and not let him out often enough or long enough. Harley took longer to break and is not 100% broken. He likes to go #2 every now and then, but at least it's solid and can easily be picked up. We had a few times he'd get up in the middle of the night, we wouldn't hear him, and he'd go into our closet, and pee on the pile of WHITE clothes. Always the whites. At least they could be bleached. We started shutting the doors and solved that. We now even keep the bedroom door open, tell the boys to get in bed, cover them up and they sleep, sometimes 12 hours with us. Despite being Chihuahua's they sleep probably 20 hours a day. They love having covers over them. Occasionally they will sleep in our bed. My fiancé & I just got back from a trip to D.C. and my parents had to watch the boys for 5 days, and they did well. They did as we do at home. Harley pooped a couple of times and peed once... on my mom's bathroom rug... were Chili always use to pee... but otherwise, they were fine. Harley enjoys barking at anything, including barking and chasing my brother's 70lbs Black Lab around the house... and birds! Silly dog.
Good Luck!