I always try to ensure my surfaces are flat and without any high spots by rubbing on a surface plate. A smear of vasaline (similar to oil) on both faces is enough to swell the gaskets but, if there are any imperfections, low spots, signs of corrosion or pitting (as sometimes evindent on magnesium clutch covers), i do us a gasket sealer as well (Loctite Master Gasket 518 or ThreeBond) just to make sure.