The book found at: http://www.geocities.com/HotSprings/Spa/7244/master_cleanse.html
recommends you do it at room temp/slightly warm. I buy boxes of bottled spring water in advance and do it at room temp. Every now and then, for a change, I drink it cold. It really doesn't matter, although it is always healthier for you to drink liquids at room temp because it takes less energy for your body to change the temp of the liquid before incorportating it into the body. Most atheletes drink water at room temp because it gets absorbed faster that way.
Other people have also mentioned making popsicles out of the lemonade as a treat.