I use a laundery marker made by Sharpie - hate the smell - but it works well.
I also have the tape and pen sold in sewing stores, got miine at JoAnn's, but have never used the tape. The pen is ballpoint and works well on the size content tags in the clothes.
For dark tags you might try a fabric decoration pen. They come in light colors. Look in any craft store or the craft section of the supermarket/kmart...
My beef is my kids have "lost" several items, including a new $30 (retail) jacket, all clearly marked, and no one ever turned them in to the lost and found!!!