I replaced the thermostat and waterpump at the same time, and I tried to clamp the hoses, but it didn't work that well. I was using these clamps from sears:
http://www.sears.com/shc/s/p_10153_12605_00947051000P?prdNo=1&blockNo=1&blockType=G1
They weren't big enough to effectively close the large lines going to the radiator. They are not marked with a size, but the jaws are a hair over 2"
I ended up draining everything, because coolant was dripping on my face.