Cooling Vests will keep you cool, but different vests are
suitable for different situations.
1. Evaporative Cooling Vests are best suited for use
outdoors, and on days when humidity is not to high.
These vests work on accelerating your body's natural
evaporative cooling. The vests are soaked in water.
As the water evaporates your body temperature is
reduced.
These vests are relatively effective. They are cheaper
than other vests. Prices start at about $30.
On the downside they are not suited to being worn
underneath other clothing. This will slow down the
evaporative effects. Your clothes will be wet after
wearing these vests. 
2. Phase change vests these vests use packs that you
freeze. The pack is filled with a gel made of Hexadecane.
The pack melts at 18 degrees. The high melting point is
very effective at keeping your body temperature at a
natural level. These vests are very effective indoors and
outdoors. They do not really on evaporation to keep you
cool so are fine to wear under clothes. You will stay dry.
The only problems with these vests is that they are
expensive $180 plus. In addition to this you will need two
or three sets of packs to get you through the day
costing around $100 plus per set. Phasechange vests
3. Personal Ice Cooling System (PICS) are the most
effective by far. They are also the most expensive about
$1800. They work by pumping iced water around a vest
using a small pump. The pump is powered with rechargeable
batteries. The water is cooled by ice packs. One of the
advantages is that it uses regular ice in packs and as long
as you can find a freezer or ice your in business.
These vests will keep you cool in any conditions. They are
even used by the military. They are suited mainly for
working environments in the heat. They require a type of
back pack of tobe worn. This system weighs of approx 8lb.
They are very effective though at keeping you cool in the
heat.
4. Cooling bandanas and neck ties use highly
absorptive polymers that are sewn inside neckties and
bandanas. When soaked in water they absorb water and
swell up. They are then worn and help a little to keep you
cool. They use evaporative cooling. They can also be
placed in the refrigerator before use for added cooling.
5. Lastly guys as simple as it may seem try to have plenty
of cold water handy. If you drink water before you start to
become dehydrated you feel much better.