Maybe just a little trick, known by many... but I just discovered it last week.
When it's hot, the inside of my motorcycle jacket sleeves get all wet, by perspiration. And also the rest of the jacket, of course.
Now, I just tried something very simple and I'm happy with the results.
I have a model of gloves that extend about 4 inches (10cm) past the wrist, that prevent any cold air to get inside the sleeve. Practical with cold weather, not so much in summer.
So, I removed my gloves, and untied the velcro closing at the wrist, so I then had a large opening around my wrist.
Then, while riding my ST, I could feel the cool breeze rushing inside my sleeves and inside my jacket.
Wow !
Plus, I unzip my jacket at the chest level, keeping the velcro at the neck. So, I stayed much more dry (I guess the perspiration could now evaporate) but it was also really more comfortable.
For my gloves, I will buy a pair of short ones, that finishes at the wrist.
Again, really simple, but being a relatively new rider, I never tried this before, and I found it really improved my comfort.