The link to the IBM article that Ignacio posted is important.
What it does, which can seem counter-intuitive, is break down "Hot" into two distinct ranges. Basically they are below about 95F and above 95F.
At the lower temperatures, maximum airflow and drinking a lot of water will keep you safe from harm. It might not be comfortable, but you'll survive just fine. You should be peeing at the normal rate, and it should not be brown! Sorry for the graphic image. Any unusual bathroom pattern usually means you are not drinking enough.
Above 95F it gets a little more dangerous. At that temperature, that "refreshing breeze" is not cooling you down, it is making things worse and can bring on heat-stroke very rapidly. You might not even notice. Again you need to keep up the fluid intake, but you also need to close all the vents and get out of the breeze. If you have a mesh jacket, fit the waterproof liner. Yeah, I know ... it sucks but it is better than dying.
A couple of years ago I rode in a rally where the ambient temperature hit 117F. I was drinking about half a gallon of water an hour, and not requiring extra bathroom breaks. It can be done, and done safely, but you have to pay attention and be aware of the ambient temperature.
Cooling vests, wet neck thingies ... I'm sure all that stuff can make you feel better, but they are messy and difficult to use and their effect is limited. For a relatively short ride they probably help, but a 5 pound bag of ice inside your jacket helps more
In the end, there comes a point where NOT riding is the safer choice, and that is an individual decision. I find it helps to always keep in mind that getting off the bike and finding AC is a reasonable choice.
ps ... Having a wife riding pillion and pouring water down the neck of your jacket is a wonderful thing