1. Reasonably easy to store and durable. So not something that demands specific criteria like being stored in very cold or hot temperatures, or that's super fragile. I don't expect it to be able to withstand a bomb being dropped on it, but I'd appreciate not needing to worry about it getting dusty or hot or cold or wet or accidentally knocked off a shelf or getting too close to a magnet or whatever. (Obviously that's a hard set of things to be completely proofed to.)
2. Able to be forgotten about. I know me and know the odds I'll forget about it for a while at some point are fairly high. I don't expect it to last for 1,000 years and be just as good as it was originally, but something that decays if it's not used wouldn't be ideal (but I'd take it if needed).
3. Cheap. Obviously. It doesn't need to be as cheap as the random stuff you get on Amazon (the first result for USB flash drive is 9 cents a gigabyte), but it shouldn't be super expensive.
4. Widely available/supported (and likely to be supported for a while). So not floppy disks or DNA storage.
I'm not planning on storing things for 1000 years or anything like that, so it's fine if it decays slowly, but it should be resilient for a while--over a decade at least.
I'm aware those are kind of conflicting, but I wanted advice on what you think would work best.