A big part of prepping is accumulating lots of stuff. It is not everything but it is a pretty major part of what we do. I am going to talk about some simple strategies to accumulate needed stuff.
First lets break this whole thing down a little. To simplify things a lot there are two kinds of stuff, single use and durable goods. A rifle cartridge is single use while the rifle is durable. Significantly different strategies can be used for these types of stuff.
The main way I stock up on durable stuff is by holding onto the older stuff I have. A pair of pants that are a bit worn is far better then no pants. Wrong color, odd fashion from a couple years ago, whatever. Still a lot better then being neikid. An older beat up pan that have been replaced becomes the camping pan. I have bunches of blankets because I never throw them out. Some are odd sizes or colors but they would keep me warm.
Single use stuff can not be accumulated in the same manner because once it is used it is done. Can't eat a snickers bar twice. This stuff must be purchased in quantity prior to needing it. Pick up 6 spare chap sticks so when I need one it is in the medicine cabinet. Keep extra cold medicine lying around because you never know when someone will get sick. A few bucks worth of OTC stuff makes getting sick a lot less crappy. What I like to do is just get two when I need one. Sometimes getting more then that makes sense.
Durable stuff can be purchased in advance if you desire to do so. There is some wisdom in this plan. I particularly like the holding onto older stuff plan because it is free. You will not care if an older pan has some scratches when the other one has a hole in it. Some combination of purchasing stuff in advance and holding onto older stuff is probably the best way to go.
The big thing is to KEEP EXTRAS OF STUFF YOU USE. You do not need twelve of everything in Cheaperthandirt, or Sportsmen's Guide. Picking up a spare jacket at a good price though it isn't something you would normally wear isn't a bad thing but you probably don't need to have fifty of them. Ditto for 1940's era Swiss mess kits, it is great that you get a discount for purchasing 12 or more but that doesn't mean you need twelve. Aside from a few spare sets of unfashionable work clothes, footwear and some tactical gear (spare holster, some sort of load bearing gear, etc) if you do not currently use it then there very well might not be a reason to own it. Do however keep in mind how your needs would change if modern utilities were nonfunctional. Can't just flip on the light if the power is out so you might want some flashlights and a lantern or two.
So hold onto your old stuff and pick up extras of disposables.