A good trick for gaskets and rubber trim in general is to apply ArmorAll once or twice a year. It's not the only product, but it is a good one. Rubber and plastics all have volatiles in them, all oxidize, and the treatment replaces the volatiles and seals
the surface to prevent oxidation which degrade them. Keeps them soft and working way, way, longer.
Just don't get the stuff on painted plastic surfaces, eventually it can loosen the bond with the paint
(i.e. painted vinyl trim) and it has been accused of loosening glued seams in inflateables as well. AFAIK not proved--but certainly possible.
Much cheaper than replacing gaskets, too.