A major challenge in rice (Oryza sativa L.) production is to enhance water use efficiency (WUE) and maintain or even increase grain yield. WUE, if defined as the biomass accumulation over water consumed, may be fairly constant for a given species in given climate. WUE can be enhanced by less irrigation. However, such enhancement is largely a trade-off against lower biomass production. If WUE is defined as the grain production per unit amount of water irrigated, it would be possible to increase WUE without compromising grain yield through the manipulation of harvest index. Harvest index has been shown to be a variable factor in crop production, and in many situations, it is closely associated with WUE and grain yield in cereals. Taking rice as an example, this paper discussed crop management techniques that can enhance harvest index. Several practices such as post-anthesis controlled soil drying, alternate wetting and moderate soil drying regimes during the whole growing season, and non-flooded straw mulching cultivation, could substantially enhance WUE and maintain or even increase grain yield of rice, mainly via improved canopy structure, source activity, sink strength, and enhanced remobilization of pre-stored carbon reserves from vegetative tissues to grains. All the work has proved that a proper crop management holds great promise to enhance harvest index and, consequently, achieve the dual goal of increasing grain production and saving water.