Good thread - your first point of call should be the excellent erowid faq on bad trips.
www.erowid.org
Ok, my opinion of wigging is pretty harsh, but if you can't do the time, don't do the crime - eg don't take more than you can handle. I know it's blunt and perhaps not the most compassionate thing in the world, but i wouldn't let myself get into that situation and i expect nothing less than that from others, but that isn't always the case.
A good trick is the "its only the drugs" mantra. I know it's kind of a cliche, but, there have been times when i have had a LOT of psychadelics, and well things get a little twisted an pear shaped. Suspicion (my friends really just use me), paranoia (man i swear to god, X is just waiting to fuckin hit me), aural + visual hallucinations (eg are they laughing at me? - is that a cop car? - is my phone ringing? etc), etc etc come into play, and reality can be hard to keep a hold of.
Its only the drugs works.
Unfortunately i'm not one for random helping of strangers who wig - got a bad pill - where's the tester? why eat 4? why sit in the middle of the rave looking absolutely bollocked? Having a bad trip - why eat so much acid? Why not have your head sorted before psychadelics - or dont take them? etc.
It does make me sound like a prick, but everyone has issues, i prefer people to deal with these themselves or with friends/etc before drugs, not getting psychosis and bringing a big melodrama to the fore because you ate too many drugs.
Gah - i'm sounding like a prick - but main pointers:
Bad trip
* keep them talking and calm, even if its about mundane shit
* keep warm, secure (eg in a blanket somewhere nice and quiet)
* keep positive
I'd be there in a second to help my friends, but as for strangers, I don't know.