A High Prairie, Alberta, viewer offers this:
...here's a little tip that I know works. If you just got a rock hit to your windshield you know that it's going to spread. If you have clear nail polish just dab a little to the rock hit and it won't spread. I've tried this and to date, even through winter, the crack hasn't spread.
H-m-m-m-m, that sounds like it might work. However, I'm more concerned with where you got the nail polish! That could be a hard sell for the wife...convincing her you need her polish!