From what I've read, the best way is by working in a pub, but personally I've never really understood how this feature works, so I don't know.
Scratch that, according to Wiki:
Confidence - The most controversial stat in the game due to misunderstanding. Sammy’s confidence is based on how happy Sammy was for the past hours, so it is a stat that is NOT MEANT TO BE CONTROLLED. If you try hard to control, you will be rerolling situations a lot that can be unenjoyable.