Believe it or not one person reported to me that just writing on an old piece of vinyl siding with a pencil held up better than anything. I used to work for a guy that made all of his plant tags using an indelible pencil. And yes they still make them.
Other people cut up old mini blinds and write on those with a china marker, also known as a grease pencil. China markers work too, but they can be hard to write with because the line is so thick.
You can also buy paint markers. Instead of ink they contain some kind of paint.
Another option is using aluminum tags. When you mark the tag you actually make an impression on the aluminum. Many backyard growers print tags on a laser printer and there are software packages available that allow you to make plant tags.
Mike McGroarty is a Garden & Greenhouse contributing editor, the owner of McGroarty Enterprises and the author of several books. You can visit his website at Freeplants.com and read his blog at Mikesbackyardnursery.com.