  A conjecture of Manin predicts the asymptotic distribution of rational points
of bounded height on Fano varieties. In this paper we use conic bundles to
obtain correct lower bounds or a wide class of surfaces over number fields for
which the conjecture is still far from being proved. For example, we obtain the
conjectured lower bound of Manin's conjecture for any del Pezzo surface whose
Picard rank is sufficiently large, or for arbitrary del Pezzo surfaces after
possibly an extension of the ground field of small degree.
