Heat Adaptation & General Toughening (Without Major Burns)

It was one of the hottest days in Republic's history, possibly even record-setting, though I have not yet seen an official reading. The thermometer at work topped out at 110 and it was still 109 when I clocked out. I could have huddled in the air-conditioned office a little longer and waited for it to cool down a bit, but where's the challenge in that? The human body is capable of remarkable things if one is willing to push through the discomfort and force it to adapt.
NOTE: Do not try this at home! I'm what you call an "expert" (or a lunatic). Gradual build-up is the safe and effective way to work on adaptation - whether to heat or cold - and toughening.