[UPDATE: Be sure to read the comments on this post; several parents suggest the Mighty Tite can actually overtighten the seat belt and be unsafe. — Ed.]
There was a time when I could never seem to get my son's car seat fastened securely in any car that wasn't my own. This was in the days pre-LATCH, when all we had was the shoulder belt, the locking clip, and a prayer. (Those days are gone; my daughter rides in a Britax Marathon fastened with LATCH connectors and a tether, and the thing doesn't budge.)
If you're still driving an older car without LATCH connectors, consider further securing your child's car seat with a Mighty Tite. This ratchet-like device goes on quickly and tightens then locks the shoulder belt in place — and the resulting fit is solid as a rock. I had the opportunity to watch the Mighty Tite in action (my cousin Hayley won't go anywhere without it) and I was mighty impressed.