Forward declarations vs includes in headers

What should be included and what can be forward declared in this header ?

class Polygon : public Shape {

public:
    void setBasicCurve (const Bezier& curve);
    void magnify (Vector direction);
    void setPoints (const vector<Point*>& points);

private:
    Plane*             m_surface;
    std::list<Point*>  m_points;
    Color              m_color;
    Bezier&            m_basic_curve;

    static Type        m_shape_type;

};

Here are some practical tips to keep your header files lean.