> module Interface
>     def some_method
>         raise NoMethodError, "Must be overriden!"
>     end
>     def another_method
>         raise NoMethodError, "Must be overriden!"
>     end
> end

Another way, which I personally feel is neatier (if nothing else because 
the error message comes a lot earlier) is to check in the constructor:

Class base_class
    def initalize
       raise 'Method required: MethodA' unless (respond_to? :MethodA)
       raise 'Method required: MethodB' unless (respond_to? :MethodB)
    end
end

It will break if the constructor is overridden without calling the one 
of the superclass though.