Ask. Nicely, of course.
You’d be surprised at how many people simply don’t ask for the order. They bury it deep inside copy, or in a weak image . Your call to action has to be loud, clear, and easy.
Something like “Pick up your phone and call 1.800.123.1234 now to order your own two week supply” makes it clear what the reader needs to do.
One particular area where I’ve seen weak calls to action is in political campaigns. In fine print, at the bottom of a postcard, is where the campaign genius decides to put the instructions on how to show your financial support.
You have to be clear on what action you want people to take and how they should take it.

