for adutls it seems communicating clearly what you want them to do will do the job. make sure you are firm, nice, and never make the relationship with them too friendly. always make them feel you are their teacher, not their 'homie'
kids: looks like a firm look can do, some compliments in front of parents will motivate them, threaten do make them do pushups, and actually make them do it if they do not listen. try to bend down and talk to them one-to-one in a friendly way. with kids good luck!