My dog Zeke LOVED the trailer when I took him on the Katy Trail. I never even considered getting him used to it, because he's such a good car rider. He "got it" immediately. Letting him run with me (the trail is deserted on a 100* week day in August) was the most fun ever. When he got tired, he'd ask to get back in the trailer. Awesome.
(I used a baby trailer, by the way.)
Now that I have a second dog who weighs twice what Zeke does, I don't think I'll be towing any dogs any time soon.