First of all I would be up front in the welcome letter. Use a gentle word of caution that for the average person wealth does not come overnight.
You might also issue that reminder in your mailer from time to time. Use the LaunchPad lesson on this subject as a reminder. This is a marathon, not a sprint.
Finally, be sensitive to those that express frustration in their emails. Sometimes a simple word of encouragement is all that is needed.
