Clean surfaces and flat surfaces. Most gaskets are soft enough to seal minor imperfections in their contact surface but if your oil pan and valve covers are stamped steel, it's a good idea to take a straight edge to them. Especially at the bolt locations. Too much tightening over the years causes the holes to distort like an inverted funnel. The result is that it's not a flat surface anymore. Tightening without a crossing pattern can warp the pan rail. It may be very slight but it will show up eventually.