Create a marketing catalog.
Buy yourself a couple of 3-ring binders and a bunch of those heavy duty sheet protectors and put two samples of each marketing piece you ever make into those binders.
This will be very helpful when you need to remember how large that postcard you sent last year was. It will be helpful as something to show a prospective new designer/marketer so they know the history and some of the jargon your company has used in the past. It’s useful as a ‘before and after’ example of how your marketing improves over the years.
If you have a designer/marketer in your business, make sure they have their own personal binder as well because they will want to take that with them and it will be much better if they don’t try to pilfer the company’s copy.
As for emails, newsletters and blog posts you can use your own discretion on whether to add these, but generally more is better than not enough. Sheet protectors and binders go on sale a lot so this shouldn’t even be an expensive project. It may take a bit of time at the beginning but it won’t take much to upkeep it and it’ll pay for itself over time.