Tell the speakers why you liked their talks! And ask them about things you didn't understand.
Same with bands. "Loved your set" is one thing, "loved your set" because "it reminded me of…"/"made me feel…" is another.
The other week I said to a singer/songwriter that I had enjoyed her full band set the day before and that her guitarist had a really surprisingly dirty guitar sound, and she said "I know! and from such a clean-looking gentleman! I'll tell him that, he'll love it". We then chatted about how her music was (for philosophical reasons) not "guitar solo" territory but how inventive and supportive his playing was in that context and I said how well I thought she'd woven it all together.
The energy you bring to putting yourself in front of a crowd really in any context, whether professional or entertainment, formal/informal, planned/improvised, you take in part from the feedback from previous attempts; it's difficult to do it otherwise.
Wrote to a composer a few years ago and pianist recently with pretty focused things I wanted to compliment, and they both clearly appreciated it because they sent very long, considerate responses (longer than what I had sent initially!)
Seriously it makes their day. They’re people too!
Some years ago I was trying to track down a comedy TV series I watched when I was a student, along with most other students at the time, which wasn't big enough to have been later issued on DVD. I emailed the agent of the actor/producer who starred in it and he gave me his mobile number and told me to give him a call. We talked for quite awhile about the history of the series and the fun they had making it, and he said if I couldn't find a copy anywhere (I eventually found it in a film archive) I could come round to his place and search through his garage because he had video tapes of it in there somewhere. He was really cool about the whole thing, and seemed happy that his work was still remembered 30-odd years later.
I have corresponded at length with composers who seemed to appreciate the interest. On the other hand, I know a composer (not even that famous, just successful enough) who has his copyist answer all correspondence with stock replies, since fans don't do anything for him and just annoy him; they aren't part of the ecosystem of commissioning works that is how he gets paid.
John Paul Hayward and Trevor Alan Gomes are fantastic musicians and have done well for themselves it seems but I highly doubt they have staff for that!
I had this experience some years ago writing to an author, too. Long reply back.