I've been instructing (on helicopters) very part time for less than a year, so haven't had time to get bored...but I don't think I ever will. I LOVE trial lessons. Just being able to put that ear to ear grin on someone's face is fantastic. It also brings back the Wow! factor for me, since you do get a bit jaded over the years, and forget just how great it is to be able to fly a helicopter.
What do I do to make them interesting? I use the briefing to find out a bit about the person. Do they want to know how a helicopter flies, or will talk of lift and yaw and so on just make them feel confused and intimidated? Are they nervous, even terrified, or can't wait to get flying?
When we actually go out to the helicopter, I let them follow me through on the controls from the start, unless they quite obviously don't want to. I talk through take-off, hovering, transition to forward flight and so on. This means that even though they may only get to attempt to fly straight and level - all you can fit in during a half hour trial in a highly unstable machine - they feel (hopefully!) like they've done more.
I think it works - ear to ear grins at the end are fairly universal! But like I said, I'm far from being an expert yet.