in general there are a few type of MOS transistor . for example in 90nm process from TSMC you can find Standard or General, Low-power, Low-voltage, High-voltage MOS transistors. if you need low power design you should use LP MOS transistor, this type transistors have a high Vt and low leakage currents, but need to keep in mind that LP transistors are low speed. this is one way to decreasing power consumption. and please remember you CAN'T use different type transistor (LP, G etc ) in same design, because they have a difference core supply voltage, and in physical design you need to separate different transistor from each others with guarding which will increase the area.