Another option is to use sodium hypophosphite, is relatively easy and at low temperature. I send you a reference:
Lee, Y., et al., Large-scale synthesis of copper nanoparticles by chemically controlled reduction for applications of inkjet-printed electronics. Nanotechnology, 2008. 19(41)
Citric acid is a better reducing agent than ascorbic acid... So if ascorbic acid is able reduce copper (i am assuming by Samy's answer), citric should also be able to reduce it.
Can you describe your problem more clearly. Citric acid, ascorbic acid are all green reducing agents. Apart from that, sodium borohydride and hydrazine hydrate can also be used.