As with documentation or the use of source control, this is a case where you need to measure and report on the behavior you want to incentivize.

I once worked for a group that was required to measure the way they spent their time, and most of the group members were 3-6 months behind on reporting. I was tasked with getting everyone 'current' -- a thankless job requiring some rather Orwellian adjustments of history. I hit upon the happy idea of offering gold, silver, green and blue stars for those who complied with the requirement to various degrees, and published the names (and awards) in our monthly newsletter. It was a silly thing, but people actually took a childlike glee in getting more stars than the guy in the next cube.

Now I measure POD coverage and compliance with our Subversion repository, and I have a little report on our departmental webserver that shows where everyone stands with respect to those metrics ... at the end of each quarter we compete with one another to see who can get a better data point on the graph. If I were in your shoes, I would do a similar thing with test coverage.

Most of us didn't get enough gold stars in grade school ... don't underestimate this powerful (and lighthearted) motivational tool.