"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."
    -- Hermione Granger

Pronunciation list:
Pronunciations for spells are included in the Encyclopedia of Spells.

Where possible, these pronunciations have come from JKR herself saying the names.
Alastor: AL-as-tor
Bartemius: bar-TEE-mee-us
Beauxbatons: boh-ba-TAWN
bezoar: BEE-zor (although Jim Dale pronounces it beh-ZHWAH)
Draco Malfoy: DRAY-co MAL-foi
Durmstrang: DURM-strang
Dursley: DURZ-lee (not DURSE-lee)
Firenze: fye-REN-zee
Fleur Delacour: FLUR deh-leh-COOR
Hermione: her-MY-oh-nee
Olympe Maxime: oh-LIM-peh max-EEM
pince-nez: pans-nay
Remus Lupin :REE-mus LOO-pin
Rubeus Hagrid: roo-BAY-us HAG-rid (NPC)
Voldemort: VOL-duh-mor (the final 't' is silent (NPC))
Weasley: WEEZ-lee

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.