This one can be somewhat difficult to judge, since colloquial expressions by nature are usually regional and often mean different things in different areas, or are worded differently in different parts of the country. Try to make sure that folksy, colloquial expressions, when you use them, are not only universally understood, but understood in the way you want the customer to understand them. They should also be simple, with only one meaning within the context of your letter.
For example, to encourage a prospect to be decisive, the admonition that one must “strike while the iron is hot”, is pretty much universally understood.
On the other hand, to say, “Life is like a sled-dog team, if you’re not the dog out front, the scenery never changes”, isn’t only a regional saying, but one that carries all sorts of other connotations and directions of thinking that will only get your customer off-track.
Use colloquialisms sparingly, and they’ll help you drive home your points. Sometimes you may want to briefly introduce a fictional character like, “My old Uncle Fred used to say…” to express the phrase. You don’t want a cast of characters, though! Uncle Fred may be remembered again at the end of the letter, reminding the reader of his sage advice as you bring them to a close.
