My dad told me to make sure everything is spotlessly clean first. Then he said put a thin film of gasket sealant on the side of the gasket that is on the part and not the engine block, that way if you have to clean it off, you're not scraping in some odd angle under your vehicle. You can always see better at the workbench compared to hanging upside down under the truck. And make sure to torque everything down properly and it shouldn't leak. He also said that it doesn't hurt to put some on both sides if it is a high stress area or where two gaskets meet.
