A short but insightful article over at Ask the VC blog outlines some best practices for first-time entrepreneurs on how to build credible financial projections.   You really should click over and read the full post, but here are the highlights:

1. Accept that your revenue side will probably not be correct, but the cost side needs to be appropriate
2. Don’t assume a big revenue ramp to be supportable
3. Define the “next stage” of your business, what it will cost to get there and what it will look like
4. Be up front with investors that this is your first time and you are looking for feedback
5. Don’t be afraid to ask for help

Good Luck!