How tight should a spark plug be?

[Tom Duffy <tomuo@pacbell.net>]

I'm trying to replace the plugs on my '68 1600, but a short try
with a small T bar type spark wrench failed to budge #1. I'm
wondering how much pressure I can put on them without
damage occuring anywhere. Should I try spraying some WD40 
to soak into the threads area? What is your experience
in removing old plugs?  
The Engine runs fine now, so I don't want to try anything
that might put it out of commission for a while, seeing as
it's my daily drive (CA, Bay Area).

Thanks,

Tom.