This may mean manipulating your local training environment with extra layers or with reduced air flow over the skin (indoors), however great care must be taken and acclimation should be a gradual process. Excessive sweating should never be a performance goal, and you should not exceed the projected conditions you will be racing in.
Clothing selection is another important factor in heat loss. Newer micro-fiber clothing creates a greater evaporative surface by "spreading out" sweat and may be more effective than training without a shirt. Lightweight, breathable fabrics are most desirable for dissipating heat and light colors reflect heat whereas dark colors absorb it.
A point of greater heat dissipation is the scalp and neck. Headgear will retard heat loss in these areas. A visor will be preferable to a hat in hot conditions.
For triathletes, cooling the body on the run should be of the greatest concern; however, a long slow climb on the bike may produce similar conditions. Interventions such as cold sponges, icing, and of course ingesting cold fluids will help cool the body to a modest extent. Muscle heat generation increases with exercise intensity and is probably why athletes overheat close to the finish with their final kick.
Drink Up
It is obvious that fluid and electrolyte ingestion is tantamount, however consumption is highly individualized. Heart rate increases with a drop in blood volume as the body must work harder to circulate a more limited amount of blood to cool the body.
For some athletes, ingesting enough fluid to offset loss is impossible. Electrolytes are the gate keepers for fluid movement within the body, and without adequate intake the gates lock up. Sodium, Chloride, Potassium, and Magnesium are the most crucial electrolytes to replace with the greatest emphasis on Sodium/Chloride.
It is important to note that hypernatremia is rare whereas hyponatremia (low blood sodium) is of far greater concern, especially for beginner athletes participating in ultra-distance events.
Understanding that you may have a tactical advantage/disadvantage in certain climactic conditions is a very important step to planning your race season. Once you have chosen your races, compare your local climate with the conditions in which you will be racing. If arriving a few weeks early is impractical, interventions such as exercising in the hottest part of the day may help prepare the body for what it will face on race day.
Review the course, especially the run. Is it windy, shady, or open asphalt? Choose your clothing carefully and have a detailed hydration/electrolyte supplementation strategy planned out according to your individual tolerances. Heat may be the toughest competitor you face on race day, but with a bit of planning you can greatly mitigate the effects.
Find and register for your next triathlon.