Just wear clothes that deflect the heat. It helps. Drink plenty of water and try to keep in the shade if you can. Don't do your shopping or whatever in the hottest part of the day, and most of all, don't over exert yourself. Have a great time!
As Ethel says SKA it's not really humid in Italy , nice and warm though . Just remember your sun block and a hat if you can , or keep to the shade if you are going to be walking a lot or standing in sun for long , you don't want to spoil your time there with sore sunburn.
The slip slop slap rule .:-) xxx