i buy my markers from paw paw. they're in the watchdog. they last forever. i only tag the roses though. all of my plant names are kept in a journal, handwritten, kind of in the order where i have them on the property and whether or not they are potted. the ones that get moved all the time are on a separate page.
Use older style aluminum venetian blind slats if you can get your hands on some.
They're wider, so there's more writing area, and they will last for years.
Use permanent marker to write with, spray with exterior sealer.
I use Eon tags, and I use a Brother label printer with their special laminated labels which hold up to sun, rain, etc to print the labels to stick on the tag. http://www.eonindustries.com/catalog/
If you don't want to buy the label printer, paint pens will hold up to sun, as will pencil. Or they sell special nursery markers that are UV-resistant (regular permanent markers like Sharpie will fade in the sun as you've discovered)
There are pretty regular co-ops on the Eon tags if you want to get them a little cheaper--I think there's one going on now as a matter of fact although I don't know if it might have closed already.