Give them a very thorough briefing beforehand. Tell them in detail about safety aspects, what to do in the event of engine failure etc. Tell them about not talking a lot to distract you, and tell them not to talk at all during takeoff and landing, or if you're talking on the radio. Ask them to look out for other aircraft, and tell you if they see any, using the 24 hour clock; they'll tell you about aircraft 3000 ft above you, but who cares. Make it clear from your attitude that flying is a serious business, and they'll still enjoy it (probably more so) but be a lot calmer. And if necessary, just tell them to be quiet!
But the longer I fly the more careful I am who I take up with me. I won't take everyone's auntie and grandpa and kid brother any more. And since the time one of my pax froze on the controls, I'm even more careful about who I let have a go at flying.