Use a desk fan. Start by investing in a desk fan if you don't already have one and ensure it's in good condition if you do. Small fans not only reduce heat buildup and circulate air, they reduce air-conditioning costs and often provide a more comfortable (less noisy) space in which to work.

Dress appropriately. It's hard to look neat and professional when your shirt  is drenched with sweat. Even if you take off your suit jacket, the shirt underneath it is plastered to your sweaty back before you board your train. Wear loose fitting and comfortable clothing, especially clothes made from non-synthetic material that won't hold in heat, such as cotton. Loosen your tie and buttons around collars. 

Do important tasks first. Take care of important or difficult tasks before you get hit by the afternoon slump caused by lunch or the hot midday sun. Not only does this offer you a window during which you'll be less distracted, but it also keeps you from feeling guilty when you totally zone out in the afternoon

Adjust your work schedule if possible. The temperature may fluctuate throughout the day, making some hours more bearable than others. If a particular time of day or office space is too warm for productive work, talk to your manager about adjusting your work schedule, or moving to a different room for a portion of the day.

Keep those fluids going. Drink plenty of fluids throughout the day, and be smart about what you drink. Keep a water bottle handy all day, and don't wait until you feel thirsty to take a sip.

Avoid heavy lunches. Try to avoid hot and spicy snacks, and opt for cooler and lighter foods and salads at lunchtime. Eating a lighter lunch can stop you from feeling drowsy in the afternoon.

Take breaks throughout the day. If you are working in an office, take plenty of breaks and if possible, get up and walk around even if it's only for a few minutes. When you are concentrating on work, it's easy to forget your immediate surroundings and the effects of the heat can suddenly creep up on you.

Enjoy your free time. The constant push for productivity in today's economic environment is sometime difficult to forget, so make sure you take time to relax and switch off from work and enjoy your free time.