I've found one that I'm happy with....so far. I picked up this at Art Knapp's last year, (Perma Stake Garden Marker for outdoor use). The key is to make sure whatever you buy says it's both water and fade resistant. Some are better than others, Another option I've had good results with is a China Marker/Grease Pencil.
A......

Pencil. As far as I can tell, it lasts forever. For big thick lines, use a soft pencil.

I've been using a black Sharpie marker pen that uses more permanent ink than their regular kind. It's labeled INDUSTRIAL Super Permanent Ink, "formulated for industrial, laboratory, and commerical use". I label my orchids with it and my oldest label, about 7 years old, is still legible. The label sits within the potting media so it gets and stays wet, fertilized, etc. Labels written with a regular Sharpie fades in a year or two.

Another vote for china markers. The writing is much darker than pencil, they never fade a bit and usually outlast the tags which tend to break after a few years in the sun.

Pencil

I use the pencil shown below............

Here is a link that might be useful: pencil

Thanks to everyone.Sorry, but I had computer issues and could not respond sooner.I have read all your suggestions and will put them to use.Also found out a paint marker? Thanks again...

Scarleta I've used a paint pen also, I haven't had it on tags for than 1 season so don't know how it will stand up.

A......

I should have used something besides a regular sharpie to mark bulbs I potted last fall. All of the tags are faded out by now and I can't read them....I meant to plant them but our spring was so wet I didn't get them all in the ground.