What are some good ways to acclimate to hot weather?

For me, it’s perfect when it’s about 40–70°F (that’s 4–21°C for you Metric folks). When it gets hotter, about 80°F (27°C), I start to droop like a wilted flower. I become unproductive, lethargic, and miserable.

Because air conditioning is expensive and I can only take off so many clothes before becoming a public nuisance, what are the best ways to acclimate to hot weather and perhaps not suffer as much as usual this year? I realize weight loss can help, but I’m looking for more immediate action I can take – things that are more systemic than just sitting in front of a fan in a tub of ice water, wishing for death. Thank you.