Keep your sessions short (under 2 minutes) and use extra high value food rewards until your dog can be calm during this process. (midatlanticbullybuddies.org)
As far as «raising the stakes» is concerned, this means using high value food, like roast meat, or hot dogs. (thehappypuppysite.com)
When the dog does potty outside, praise enthusiastically and reward either with high value food (hot dog, chicken meat, etc) or a game of fetch etc.. (russiandog.net)