Ask HN: Development for small range of customers

Are there any approaches on sucessfull/efficient development of wide applications for a small range of customers? Given that there is a financially strong, but very small customer group whose business processes are based on an international standard, though differ in many significant ways. So it seems to be impossible to develop an application that can be sold to every customer without major adjustments being neccessary.

So eventually some smart guys/girls came up with an idea to serve small customer groups with maximum efficiency and minimum amount of work?

I hope I made my case clear. It was hard enough to phrase my question in my native language ;)