Put it as layout text on top of the field. Use "hide object when" (FileMaker 13) or conditional formatting (Older versions) to make the layout text invisible when the field is not empty. When you enter the field, the layout text will disappear and once you have entered text the layout text will stay hidden.
If you use conditional formatting, use a font size of 120+ points to make the text disappear if the layout text is a single row of text in the 10-12 point size range when visible.
Worked perfect, thanks!