A spokeswoman said that, although the rate at which new volunteers are recruited has been on the increase since 2003, the total number of volunteers working for the charity had fallen each year since 1993, and stood at 14,433 by the end of 2007.
The number of calls the charity has received has been increasing since 2004 to the 2007 figure of 5,319,462.
A poster for the recruitment campaign, which will be launched on Monday, picks a man out of a crowd and carries the text "Everyone has it in them to be a Sam. You've got it in you too."
Like previous Samaritans campaigns, the press, radio and poster ads will only use donated media space.
The charity is also looking for volunteers who can help with publicity, fundraising, admin and IT.