Can we define "doomsday"? I mean because in standard vernacular "doomsday" means: "end of the world." As in, nothing left exists. As in, giant meteor blows the earth up into a million pieces. As in, volcanoes, hurricanes, floods destroy everything all living life forms on the planet.
So if we are truly talking about a doomsday scenario, then there really isn't anything to prepare for. No amount of food, rations, ammunition, guns, water, concrete bunkers etc. is really going to make one lick of difference when the entire earth is 200' below molten lava and/or water, while simultaneously being pelted with meteorites and is hurling toward the sun.
Getting hit by an earthquake, being ravished by a tsunami or tornado, losing power for several days; those are natural disasters, not a doomsday. Preparing for disasters makes sense. It could save your life. Preparing for the end of the world as we know it; aka doomsday? I think the best preparation you could do is live your life the best you can now while you are alive so you don't have any regrets when the end of the world comes.