I clean all gasket surfaces with carb or brake cleaner, make sure the surfaces are straight and have no burs. Then I use top quality gaskets. I've been using the Milidon crushproof pan gaskets with good results. I always supplement all mating surfaces with Permatex Ultra Seal, making sure to massage it into the surface prior to bolting things up.

The last rear main I did in the car was the Mr Gasket silicon seal set. I believe only the side gaskets are actually silicon, but they work nicely. Attention to detail, cleanliness, Permatex Ultra Seal in the correct amounts and slightly off-set the seal. No leaks.

I did have to slide the aluminum seal holder in and out a few times to get the side gaskets just in the right spot, so they were not sticking out or sticking in to far.