If you're mashing in the oven, a pretty standard way to do it is to preheat your oven to 170 (the lowest temp for most ovens), heat your mash kettle to 150-something on the stovetop, stick it in the oven and turn off the heat. The oven will cool off faster than it can heat up the water in the kettle. Water has a very high specific heat capacity, and will take a pretty long time to change temperature. You can check your oven and reheat to 170 as needed.
One thing to consider is that enzymes denature at different rates at different temperatures, and the rule of thumb I've heard is that they last about an hour at 158, You could always add more, though. I usually use a really long mash just because I figure it's only going to help.