Work backwards.  Determine what your profit margin is on a sale and then determine what percentage of that profit you're willing and able to dedicate to acquiring a new customer.   From there you can create a marketing plan and investment strategy for generating new leads and converting them to customers.

If the question is "how can I get new customers for zero dollars" then the answer is simply: you can't because your time is worth *something*.  So part two is figure out what your time is worth and compare that to the investment you determined in the first part.  If your time is less than the cost to generate a customer in a traditional marketing strategy - and if you have the time to spare (and only if you have the time to spare) - then you can consider a DIY marketing approach.  If your time is more valuable than the customer acquisition cost, outsource the marketing to someone who can do it for less or provide a better service thus increasing your bottom line.