Here are some articles and opinions that you can read on the web:
- I have a long post on my blog http://blog.timberry.com/2007/01/q_a_selling_out.html
- Small Business Trends recently recommended bizbuysell. Here's that article: http://feeds.feedburner.com/~r/SmallBusinessTrends/~3/149378140/value-your-business-in-ten-minutes-at-bizbuysell.html
- There are a lot of other possibilities out there. Do the web search.
Sometimes people will do a business plan to determine future cash flows as a means to develop a valuation analysis. The business plan helps you think about the value as a matter of future cash. Sometimes the sellers will offer a business plan, and in that case you have to look at it carefully, with a healthy skepticism.