For the Hottest Coffee, Preheat Your Mug
Preheating ensures that most of the heat stays in your drink instead of being absorbed by the mug. To prove this, we conducted a test kitchen experiment.
If you like your coffee piping hot, it's a good idea to preheat your mug: Fill it with boiling water, let it sit for a few minutes, dump the water, and fill 'er up. Preheating ensures that most of the heat stays in your drink instead of being absorbed by the mug.
To prove the point, we monitored the temperature of freshly boiled water (212 degrees) that we poured into two identical ceramic mugs, one preheated and the other not. After 1 minute, the water in the preheated mug was 15 degrees hotter than the water in the unheated mug. After 10 minutes it was still 10 degrees warmer, and even after 25 minutes, it was still 5 degrees warmer. Preheating works well because ceramic has a significant amount of mass that can absorb and retain lots of heat. (Note: Don't bother preheating a double-walled carafe. The airless layer between its two walls prevents the transfer of energy, so hot liquids stay hot without any extra help.)