Write a short example or explanatory sentence, and put it below or
beside the text field. Two examples conjoined by "or" works fine
too. Keep the text small and inconspicuous, though readable;
consider using a font two points smaller than the label font. (A
one-point difference will look more like a mistake than an
intended font-size change.)
Also, keep the hint short. Beyond a sentence or two, many users'
eyes will glaze over, and they'll ignore the text altogether.
This pattern is often used in conjuction with
as illustrated by the Word example below, or Structured Format.
Alternative patterns are
(in which a short hint goes into the control itself), and
(which puts an actual valid value into the control). Input Hints
permit longer, more permanent help text than Input Prompts, but
because no default value is present in the text field, the user is
forced to consider the question and give an answer.