I read the information on the link that Red Ryder provided. It seems like a very good place to start and an intelligent way to determine an approximate starting point for tire pressures.
I have neither a skid pad nor a temperature probe. What I DO have is what everyone else does, a sense of when the vehicle is taking more steering wheel input to make a turn, or when it's starting to slide in a turn, or when it seems to wander on a straight road.
When I got my car in December '08 and for about a year after, I ran the recommended cold inflation pressure (CIP) of 30 psi in all 4 tires. I was constantly amazed at how quickly it started to slide in a corner when it was dry out, and how unsafe I felt when it was even damp, plus how imprecise the track was when on a straight and level road. On top of that, I watched as the edges of the tread continually scrubbed away. Now I'm no engineer but I'm intelligent enough to observe the difference between these tires and the new ones that I put on my previous car. I saw these (Goodyear Eagle LS2) wearing out way faster than the old ones (Big O Touring), both of which are rated at 44 psi max CIP.
Finally I decided to change the CIP from 30 to 40. I posted about it earlier. When the dealer service dept rotated them and put them down to 30 again I pumped them back up, but this time went to only 38. At 40 the ride was very harsh but cornering improved greatly. At 38 the ride is slightly better (maybe just my imagination since I know I dropped the CIP) and the cornering is about the same. I'm still in flux with what CIP I'm going to use, and 36 will be my next test. But I can definitely tell you that 30 is just too low for my car. I'll probably test using 34 as well.
The biggest thing I took away from that article is that the proper CIP for any tire has to take into account the tire construction, the outside temperature, the load on each tire, and the way that you're intending to use it. What you're trying to accomplish by adjusting the inflation pressure is a uniform contact patch, one that spreads the load and wear across the entire surface of the tire rather than impacting the center more than the edges or vice versa. If your driving is what most of us do (driving on public roads with the occasional fun blast here and there) then that's different from taking it to a road course where you're gonna be constantly throwing it around a lot of corners. I honestly don't know if you should increase or decrease the CIP for a road course, but the article tells you how to determine it, if that is what you need to do.