Often, when firefighters are in rehab, the one repeated mistake is that they are still wearing their turn-out gear. Whether it is your bunker pants or an open turn-out coat or your lid, it is all preventing you from cooling down efficiently and quickly.
Once you have entered the rehab area (hopefully you have one designated) one of the first orders of business should be removing full turn-outs and helmets. If your rehab area is outside consider ambient temperatures when doing this. Otherwise the sooner these items are removed the sooner you can start cooling the core areas that hold in so much heat.