Revenue Forecasting - Service Firms

I am looking for a forecasting model or tips on developing a forecasting matrix that can capture business development efforts, labor and backlog, current proposed work, and alliance (rarely proposed or bid on) work. Also, often there are times when existing work is increased by the client - sometimes just pushing back due date but within the same proposed budget and sometimes increasing budget, generating more revenue for the firm.

We are currently getting a weighted-revenue by using the sales reps' close probability. We then factor in the projects' start & completion date. We base backlog off of historic data, which seems to fall off at the end of the year but that isn’t accurate. And, we haven't found the best practice in predicting alliance work; but, we use various indicators to gain insight.

Are there any suggestions, templates, or resources available? Or, someone with experience in sales forecasting for a small - private professional services firm focusing in engineering, design, and project management.