There is an excellent post on from Guy Kawasaki on projections and how to go about doing them. Here is one of the practical points that he makes...

Forecast from the bottom up. Figure out how many business development and sales meetings you can get per week--that is, four or five. Then multiply this number by the percentage that will be successful. Then add six months to close the deal. This forecasting method yields a much smaller number than the “conservative” method of assuming that you can get at least one percent market share.

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