"Drink lots of water," advised the California couple I met on the Circumvesuviana train en route to Pompeii.
"Otherwise you might end up like one of those mummies!"
The ruins at Pompeii and Herculaneum were the reason I forewent Milan and Florence for southern Italy, because I felt like I would be a bad anthropologist if I didn't make an effort to see such remarkably preserved evidence of early civilization. My dedication proved crucial, as it was over 90 degrees and the stone ruins offer no shade. But luckily (at least at Pompeii) the old fountains are still fully operational with cool, clear drinking water.
While I'm glad I saw Pompeii, ultimately I preferred Herculaneum because the former felt too touristy and too big - like if you got lost you could unwittingly spend an entire day there, or perhaps never make it out... (Also I was expecting to see more petrified human remains, so that was a bit of a disappointment).
Herculaneum is a lot more intact as a site, both in terms of its physical structures and its art. The paintings, mosaics and statues retain their vibrancy despite being nearly two thousand years old, which is pretty remarkable. It also provides a more intimate experience, as there are less visitors and you can get a good look around the entire town in under two hours. I didn't even get lost once!