Soak a towel in very hot water, and then wring the towel to remove most of the water. Fold the towel and place it on top of the impression in your leather seat. Place a heavy book on top of the towel, and leave the towel in place for 20 to 30 minutes. Repeat this process a total of three times.
Treat the area with a leather conditioner to prevent the leather from being dried out by the water. Allow the leather conditioner to soak in overnight, and do not sit on the area while it dries.
Inspect the area to see if the impression is gone. If there is a remaining impression in the leather, use a hair dryer set on the highest heat to warm the area for roughly five to 10 minutes. You should see the impression disappear. Once the impression is gone, treat the area with leather conditioner again to protect it.