+1 Make sure there isn't any old gasket material left. A razor blade will make short work of stripping old gasket from the surfaces, but I will warn you that the aluminum can be taken away with a razor blade, too. Shallow angles and keeping the blade flat are the keys.
Make sure that you get all of the debris out of the parts before assembly! When dealing with assemblies with tight tolerances (engines, for example), hospital clean is a good start.
Once all of surfaces are free of gasket left-overs, I wipe the surfaces down with a clean towel wetted with acetone. This will take any oily residue away and leave you with good clean surfaces. From there, make sure the gaskets don't wrinkle up when mating the parts, follow torque specs and you should be good to go.