How can passengers help in the cockpit in an aviational emergency?
The short answer is to sit down, shut up, and stay the heck out of the way.  That's what I would do.  If you can quietly help the one or two people sitting next to you do the same, you're doing your part.

Seriously, the last thing the flight crew needs in a flight emergency is extra distractions from passengers - especially those without directly relevant experience (e.g., an off duty airline pilot commuting home).

Less seriously, it's a common fantasy/daydream among private pilots that some weird emergency results in them taking over the controls.  Pre-9/11, I had the opportunity to try landing a 747 in one of the airline's full motion sims.  (I have zero relevant experience in flying something that big).  On the first landing, all the big pieces came to a stop on or near airport property.  On the second attempt, I think I got most of the pieces to come to a stop on or near the runway.  I doubt many people would have survived either attempt.

My plan if this would ever happen in real life is to get on the radio &amp; demand that an instructor tell me which buttons on the autopilot to push.  If successful, I'd then go buy a lottery ticket before the crazy luck wears off :-)