[Yosemite Sam]

I have used dry gaskets, gaskets smeared with a very thin coat of grease and used gaskets over again, all with good results.

Within the past few years, I have grown to like Permatex® Form-A-Gasket® No. 2 Sealant. It doesn't get dry and hard, it cleans up easily and gaskets can be reused (if you're half way careful when you take it apart again).

I think it's probably more important to make sure the mating surfaces are clean and not damaged before you put it back together. I like to use the scotch-brite disks on a die grinder to clean them (but not on aluminum).