Just make it policy. If people who have never tested don't have to test, they will generally not try it, since it's bound to result in at least a temporary slowdown in work and they don't want to get blamed for that. You have to make it officially okay for them to "waste" time on writing tests before they are likely to try it. (To be honest, it's resulted in a permanent slowdown in my work, but has also reduced the number of bugs and made it easier to make changes later, so I think that's a fair trade.)