What is the most proper way to potty train a dog?

I have an 8 year old Poodle named Cocoa. He was my grandfather's, and was never potty trained. I've been working on it, but I'm not getting too far. I give him treats, and pet him when he goes outside. I try to make sure that he understands that is what he is supposed to do. I have to take him outside once every 15-30 minutes, otherwise he goes in the house. I take him out that often so he understands that's what he is supposed to do as well. Because I do it like that, throughout the day he does fine.

He knows he is not supposed to go inside. He has gotten to the point that now he uses the bathroom in the house out of spite. He will be doing fine, unless he gets upset. If he potties inside and I don't give him a treat, it causes a chain reaction for a couple of days him using the bathroom 3-4 times in 30 minutes all over the place. If I give our other dog the slightest bit more attention, he will go right in front of my door. If the other dog jumps the gate and goes throughout the rest of the house, he poops right by the gate. If a friend comes over and doesn't pet him, he'll go in the house. After 6 months of this cycle, he seems to be to stubborn to train at his age. He also usually uses the bathroom during the night. I started taking him out around 12:00 am, and 3:00 am. There is still usually a spot when I wake up. He won't do it when anyone is watching though.

My real question is, is there a better way than just giving him treats and talking to him in a sweet puppy voice when he does well? There must be something I'm doing wrong. When he goes in the house I usually tell him no in a stern voice. Should I be disciplining him more? I'm not too sure what to do.

Try looking up information on crate training. That should help you get a routine going. It will help your dog understand that the crate is his home and that slowly the dog will understand that the space outside the kennel is his home too. And dogs generally don't want to mess up their homes.

Also, you need to be vigilant all the time. If you literally catch the dog IN the process of doing business inside, then you can interrupt them. It took my dog a few weeks to get potty training, but when he did get it, then he was good. But once he got the idea, he needed a little reinforcement. I caught him peeing twice and I interrupted him with an "ack" noise and HE STOPPED mid-stream and walked to the back door. So constant watching. It helps if you keep the dog attached to your waist throughout the day on a leash, then you can see when he goes to do his business inside and interrupt him in the process.

Good luck!