My oldest was easy, we just brought the potty chair into the living room and removed the diaper, and she figured it out fairly quickly. Supposedly girls are easier than boys though.
My youngest, he has been a real headache. Having other kids around makes the whole just not wearing pants and peeing in the living room thing a bit more difficult. He's starting to get it now, at five, but it's been a long road of positive reinforcement. Finding the right treats was essentially the key, and now it's routine. Like a dog!
So I guess-
Girls: Make it convenient for them.
Boys: Train them like a dog.
¯\_(ツ)_/¯