Here is the solution, for I used it on my Targa and it worked great. Go down to your local hardware store and buy a small can of lockease. It's a liquified graphite that is used to free up door locks. Gently apply some around the fittings, so that it will 'seep' down between the plastic and the aluminum sockets. Don't use a 'large' amount, but rather start with a moderate amount.
Just leave them for a little while, and give the liquid a chance to drip down. The alchocol will evaporate, and leave a very light film of graphite there. It's likely that you will have to work with the seat to get them out the first time. My technique was to kneel down, and put my shoulder under the seat (pushing up), while working the latch.
After getting them out, clean both the socket and plastic insert to get most of the dirt/grime off. Then apply some of the lockease to both.
If you overdo it, the seats can have a tendency to come out if you don't have them locked in real good. All in all, I did this on my Targa once, and in the next two years I never had a problem moving the seats around. Only had my seat come out twice (but I'm a pretty big guy and was leaning back on the seat pretty heavily both times).
Tex