If you're talking about marking potted plants, don't use Sharpie markers, they fade very fast. I use common #2 pencil and that will last at least a year on white plastic plant labels. Recently i made the "brilliant discovery" that after marking with pencil if i push the label into the soil, so just top is showing and writing doesn't get exposed to the sun, then will last several years. Another easy method is to mark your pot on outside, no label needed, with a wax marker, like the ones called China Marker. You can get them at art supply stores. Lasts a very long time. Aluminum labels don't work well here as they soon get covered with mold, algae, and other growing things that will cover up the writing.
It seems like plastic tags commonly available now have a harder surface that does not "take" writing as well, regardless of what you use. It pays to check both sides of the tag as one side sometimes will take the writing better. The wax pencil seems to be by far the most persistent but they always seems to be fairly broad tipped which makes it hard to record much info on a reasonable size tag (writing on the pot is a good idea if not using black nursery pots). Pushing the tag down into the media greatly helps preserve the writing. The problem I find is that my plants tend to sit too long in the pots and, with repeated weeding, labels seem to somehow "disappear" (no doubt coming out with the weeds & not noticed). If you make the plastic label into a tag that you wire on, it becomes brittle & is gone far too soon.
I am skeptical about metal tags you write on -- I am sure that they would get covered up as Oscar reports. Swimmingfree's solution sounds good but also sounds time consuming. the Dymo label maker has been working well for me so far but requires a bit of a cash outlay. It embosses the letters well into the metal so, even if covered by mold, algae, etc., it should wipe off easily (& perhaps be even easier to read as a result). I have attached a couple of pics of a tag I did for some ulu I was trying to propagate. Resolution is not great as my camera died & I had to use my phone but I have attached a pics from each side to try to give detail of the letters.
John