Items: listed in priority. After boots-n-pots) I usually rush Rylai's Crystal Scepter, and start with Giant's Belt for that unexpected early game tankiness. After Rylai's, I usually get my Sorcerer's Boots and then go for a Sheen. After Sheen is built, forgo rushing the Lich Bane and start building that Rod of Ages. (*NOTE: I understand the importance of getting RoA first, given the time and leveling boosts it has on it. That is perfectly fine as well if you think that is more important the the Ability Power and bonus after using abilities.*) After these two items its fair game. I build Rabadon's Death Cap and then finish off my Lich Bane. After that it all depends on the situation (*NOTE: It ALWAYS depends on the situation, I understand this... this is a generic build to help guide people who want to try out playing a Fizz mid... and own at it.) And of course to finish it off you can get a Guardian Angel armor or another Ability Power item, if you have the gold for it, why the heck not?

Leveling: focused around Seastone Trident (D.0.T.)
Runes: Magic Penetration (Marks), Health (Seals), Ability Power (Glyphs and Quintessences)
Masteries: Focus on offensive Ability Power masteries, along with some solo-lane worthy utility masteries. They are also able to be used w/ any combination of 4 Summoner Spells:

Summoner Spells: I usually run Teleport/Surge or Teleport/Ignite, though some Fizz players don't always rely on Playful/Trickster(E) to get out of fishy situations. I recommend those type of players use Teleport/Flash. These masteries can be used with all of these combinations.