This is a common problem for private companies. The only way to get a solid-ish valuation is to have the auditors do it. It will be based on revenue, profit, balance sheet, projected growth etc. I think there's some sort of formula that they use (companies often ask them to do it in order to value share options). From your own perspective, if you do get a valuation and agree to sell, try to exclude the "minority" nature of your holding as a valuation factor for your shares and base it on a straight-line proportion basis.
If there has been recent investment in the company or if share options have been granted recently (and valued), then that will be a good starting point. If neither has happened recently, then I think you'll need the auditors involved.