All of the above, plus...
Similar to telling pax to button-up while under pressure, warn them that you're always listening in to the radio, and may need to break a conversation at any point to respond (or to listen in). 'Before I talk on the radio, I'll say "Transmitting". Everyone is to stop talking straight away'.
Always take spare headsets. I have my own, and will always take two similar ones out from the club - a spare in case of a failure, and similar in case mine isn't compatible.
Back to not rushing your pre-flight checks.... .I took my neighbor up a couple of weeks ago (only my 4th passenger, after Mum, Dad and girlfriend). Because it was just the two of us, I talked him through everything we did - from checking the TAFs, METARs and NOTAMs over the internet from home, through to signing the techlog and picking out headsets. We then booked out and did the walk round together.... and so on, through all the checks - explaining what each instrument did (basically), and what he could be looking out for (limitations) on the enginge instruments.. .This paid off when, for the return flight (another £120 bacon sandwich from Wellesbourne)... On arrival we were advised to stop on the taxiway and push back onto the grass to park, because the ground was so soft. When I tried to taxi off again, I was almost at max power, and the kite just wasn't moving. I pulled the throttle back, and began to think of my options... shutdown and pull forward, wait waggle the rudder a bit... and as I did so my pax piped up, "ummm - breaks".... .It was nice to know that he'd spotted it, and that he might just have noticed something else too...