How can I cool down a laptop power supply?

Raise it off the floor as suggested and train a desk fan on it - and/or get a higher wattage unit.

This is a fairly old question but I have a rather unique solution. Buy a good laptop fan just for the power brick, it won't provide so much cooling that it will cause damage but will keep it running at a reasonable temprature. I own both an ASUS ROG gaming laptop and an Alienware one as well. Both power supplies have heat issues.

Personaly I use a box fan leaning at an angle against the wall and I have made a support to hold the psu against the fan. This keeps it very cool.

The picture I attached is an older setup in which I used the power cables size to hold it on the fan! (also the psu has feet which fit in the fan notches

Absolutely, the best thing you can do is to allow the PSU to cool naturally, don't try to cool it quicker as doing so may cause damage (if it's not already damaged). Allow it to cool fully before attempting to use it again, and monitor your laptop and the supply when first using it in case it has been damaged.