I learned a lot during my first seat check.
1) Comment on how cute their kid is, or how well-behaved they are. Even if you can't find anything good about their seats, at least compliment their offspring.
2) Talk to the kids! Say hi, ask them how they are doing. Engage in conversation even if they barely talk. The parents seemed to like that we really cared about the kids.
3) Compliment them on their choice of seat! People really like to hear this. LOL I had about 3 safeseats and a keyfit come through so I didn't even have to lie. One guy got kind of defensive when I showed him why his seat was tilting ("But that's how the fireman installed it!") and when I complimented him on his seat and showed him why it's so great (awesome lock-off) he was very easy to work with.
4) I just tried to point out what they did RIGHT. Then non-chalantly pointed out how to fix their misuse without saying "you did it wrong". One guy came in with a keyfit and had a GREAT install, only the angle was a bit off and he was using both LATCH and the seatbelt. I told him his seat was nice and tight and I was just going to un-install it to make sure everything was where it should be. I then asked him whether he thought he liked the install better with the LATCH or the seatbelt and he said LATCH, so I told him to go ahead and re-install it just the way he did before, only without the seatbelt and also showed him where to change the recline. He got "another" perfect install and I made sure he understood you couldn't use both together.
5) Empathize with them. "I have a difficult time with this seat too" "I was confused about that also" "I think the manual is unclear sometimes too". Works like a charm.