Advice from aerobatics
I hold an aerobatics rating and as you can imagine some friends want the good old roller coster ride but end up not having the stomach for it. Few things I have learnt is if they feel any discomfort, like a previous post described, they need to tell the pilot rather than keep it to themselves. That's the worst bit because they will be thinking about it constantly and they will make themselfs worse. And the lunch will make its appearance.
Best way to deal with this try to take their attention off their insides. Engage them in pointing things out to you or taking control of the aircraft if your up for that. Just a few minutes and all will be good again. If all else fails give them the sickbag before its too late. One of the things playing on their mind making the situation worse is what if they make a mess. Giving them the sickbag reassures them that they are covered and help the mental side of things.
With regard to nervous pax, a strong confident voice is good. Explain what's going to happen before it does for example "we are just waiting for that guy to land then we can line up on the runway." As you line up, "now we are just waiting for ATC to give us the go. Ok here we go." Your telling them what is happening in simple terms without overly complicating things. Also tell them in the classroom before you go out. Explain the flight like the instructor did before your 1st ever flight.
If you do the classroom bit this is the best time just before you go out to remind them of the standard hazards like the propeller blades and knocking their head on the wing or pitot tube on the 152s
Again don't go into detail brush over it if you will. Short sweet but you made the point and it's in their mind. Happy cheery voice so it's not all down gloomy and taking the fun out of things!
Hope this helps and sorry for the essay. I got carried away!