"Her-my-oh-nee," she said slowly and clearly.
"Close enough," she said...
-- Hermione Granger and Viktor Krum
"You're saying it wrong. It's Win-gar-dium Levi-o-sa,
make the 'gar' nice and long."
Where possible, these pronunciations have come from JKR herself saying
The Harry Potter Pronunciation Table, a website that specializes in pronunciations of many, many Harry Potter words, is THE place to go if you're wondering about any other words and how to say them.
Since many of the spells come directly from Latin, the Latin pronunciations are considered to be correct. It is worth noting that in Latin, unlike Italian, does not have a CH sound, so Accio would not be pronounced in the Italian style as AH-cho and Crucio would not be CROO-cho. Because the words are Latin, the C would be pronounced as an S.
See also the pronunciation guides listed on the Web Resources page.