High performance digital circuits require long bus lines to operate at very high frequencies, necessitating a large number of repeaters to be inserted along these lines. Power consumed by repeaters, particularly that contributed by subthreshold leakage, is becoming a major consideration in digital design. We compare several threshold voltage assignment schemes to reduce runtime leakage power in buffers. We explore trade-offs between dynamic and static power by selectively mixing high and low Vt devices within a pull-up or pull-down network. We propose an activity-dependent hybrid Vt assignment scheme that can be applied across a bus. These configurations are shown to reduce total power by up to 38% and runtime leakage by up to 48%, with negligible design or area overhead.