I also use a P-touch label printer. They don't fade, and are waterproof. They last as long as the plastic tags I stick them on. They're a bit more expensive than pencil, but a lot clearer. The PT-1230PC does labels up to 1/2 inch, the PT-2430PC does labels up to 1 inch. Either one will work for plant labels.

Just as a testament to longevity, I have a L. rubescens who's tag, pencil on what I believe is polystyrene, has a first repot date of 1968.

Someone on this site told how they use stainless steel washers, a numbering system, and number/letter punches to keep tract. I love that system. You do have to assign everyone a number or letter or combo. But one washer as a tag for the plant and one potted deep in the medium is excellent for permanent id - they don't rust, fade, and if the visible washer disappears, the potted one is still there.

I don't believe the initial investment is that expensive and if you do have a large or booming collection it can really help keep the noids to zero.