Sort your contacts before writing
Split your contacts into three groups: those who asked for something concrete (a quote, a sample, a visit), those who showed interest without a request and those who just stopped by. The first group gets a message the same day or the next; the second, in the following days; the third, if anything, gets something useful, not a proposal.
At the event itself, write down one detail from each conversation, on the back of the card or in a phone note. That detail is what goes in the first line.