Great question. I think as business owners we often wonder about our business valuation but perhaps never get around to ask it.
I'm neither a qualified business valuer nor have extensive experience in this field, but I can talk to some of my personal experience and give you some ideas. In fact, I just went through a similar process involving a family business.
I used a combination of 3 methods below - initially as a back of the envelop exercise, then refine it iteratively:
1) income based - i.e. earnings such as EBITDA and cashflow calculations
2) asset based - asset values, liabilities, liquidation costs, etc. I'd argue that you should consider both tangible and intangible assets, such as IP, brand recognition i.e. assets that have net positive effect on the business long term
3) market based - basically what the market is prepare to pay, based on comparing of recent sales of similar companies. For instance, sifting through sites like realcommercial.com.au or domainbusiness.com.au gave me a ballpark idea.
I reckon business valuation is a combination of science and art. The methods I used above is merely a guide, but you will find that there are lots of intricacies in applying these methods. What I found is that different assumptions will easily throw your calculations off a tangent. This is where a good professional business valuer can really help with bringing the art of applying these and other methods based on their experience. What I found critical too is a good business broker who knows the market inside out, and knows how to market your business accordingly.
At the end of the day, you need to be confident that the right combination of techniques and experience are used reasonably to valuate your business when you're ready to sell.
Hope this helps.