I've found myself riding a lot in hot weather. Last year for example whilst crossing the Alps I had several days of over a 1500m of off-road ascent in temperatures approaching the high thirties so keeping cool is essential.
For me, one of the biggest changes I made a few years ago is using High Five Electrolyte tablets. I'll have one bottle with this and two plain water bottles and drink alternating between them as the day goes on. If it's been a super hot day then I'll also drink a bottle of electrolyte in the evening. I know folk say you can make your own, but when out touring in the heat I've found these to be a game changer, my ability to recover at the end of the day has really improved and I no longer suffer from cramps or headaches in the heat.
Another thing I do is wear Merino wool T-Shirts, I love them they are really cool, wick sweat easily and can be worn for multiple days on tour. Helmet, yes I wear one for touring of road, however on long Alpine climbs it sits on my saddle bag, I really don't see the point in wearing it on climbs as pointed out above, they can be really hot and sweaty.
Another unique one for me has been ditching the padding. I was getting terrible chafing due to Betty Swollocks and really struggled on multi day trips as well as single day rides. So I ditched the padding for Merino Wool Boxer shorts and thin mountain bike shorts without the padded liner. I can ride in multi day comfort now in really high temperatures and never suffer from saddle sores, it's bliss.