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!