Assign them to projects that don't require "top quality stable code" if that's your concern and let the jr. developer fail. Have them be the one to 'come in on the weekend' to fix their bugs. Have lunch a lot and talk about software development practices (not lectures, but discussions). In time they will acquire and develop the best practices to do the tasks they are assigned.
Who knows, they might even come up with something better than the techniques your team currently uses.
