Advice for mashing in an insulated cooler

For the longest time I had problems getting my mash to the proper temperature in my rubbermaid cooler. I would add the strike water and hit the temp right on the money only to have the temp drop below 150 degrees. I tried pre-heating the cooler and that helped but it still dropped too much. This last batch I figured it out. The cooler absorbs heat from the water. Heres what to do:

Pre heat the Mash tun with hot water. Then heat your strike water to about 5 degrees hotter than your target temp. Pour the water into the tun and let it cool down to your strike temp then add your grains.
I did a 90 minute mash this way and I only had a 2 degree drop from 154 degrees.

An easier way is simply observe how many degrees you lose because of using a room temp mash tun, and next time increase your strike water temp by the same number of degrees.

Promash has a utility to help calculate this, you just need to figure out what the thermal mass of your tun is.

Sounds like your cooler isn't insulated that well? I use a Coleman Extreme and it doesn't typically drop more than 1-2F over 60m even at 50F ambient.