What's the etiquette for following up on academic contacts?
As far as I see, these phrases mean what they say (which is not that much, but more than nothing). I also hear them a lot and say them from time to time. However, they do not promise anything else than some interest in your work or field.

When I say these phrases to people I just met it means that I found their work interesting and can imagine to spend more time thinking about it (but probably will not think about it on my own). 

Another view is: Many people collaborate with many other people and enjoy it. See this phrase is a very low level entrance to something that may be a collaboration in the future. 

More on the practical side: You may always send an email as a follow up, either with a question or something like "I enjoyed our conversation; here are links to some of my papers/other interesting stuff on the topic. Looking forward to seeing you again."