Look at other businesses in your same field/niche. Research your potential customers, take the cost of product/service over head and subtract.
It is not as easy as ShaneS might think. Simply looking at other businesses in your same field/niche or comparables of similarly situated companies does not always work. There are many factors which go into your sales forecast which are specific to what you are doing or your specific sales and marketing operation. They include achievable sales targets from your business development team and your overall marketing strategy. This is more of an art than a science and should be created by the key executive with the experience capable of a diagnostic assay of the overall plan. If you do not have this skill set yourself should find it. When savvy investors look at a Business Plan or PPM and can shoot holes in the sales forecast which in turn questions the entire financial models you will never regain you credibility.