You can reward people simply for checking in, for checking in a certain number of times, for checking in with friends, or reward the person who checks in the most (the «mayor»). (entrepreneur.com)
For me, I like working out a certain number of times a week, meditating or reading. (entrepreneur.com)
Set policy throughout the year that if a recipient does not engage a certain number of times a year you will stop emailing them. (entrepreneur.com)