many people don't understand the need for manaregen in the mid lane, if your mana can outlast your opponents, then you outlane them. This is the reason for the Chalice of Harmony.

The Mercury Treads gives you extra tankiness against your opponent.

The Rylai's Crystal Scepter provides you much needed tankiness as well as AP and a crazy slow to your enemies.

The Abyssal Scepter is a great item because Diana is a melee mage, so the reduced range of the magic resist does not affect you. However, if your opponents are AD heavy, you can change this to a Zhonya's hourglass

The Rabbadon's Deathcap gives you that final ooomph that you are looking for.

The last and final spot is a situational item. You can either take this before or after your Rabbadon's deathcap. if you are focused down a lot, then you can use a Guardian Angel, or if your team is devastated by their ADC then a Frozen Heart will be useful as it also gives you CDR.

During the laning phase, focus on farming and zoning. if you can land a "Q" on the enemy champion several times, they will be forced to return to base to heal. If they foolishly decide to stay, then you can "Q", flash, ignite. This usually kills off anyone with less than 1/3 hp in the lower levels.

Once you reach level 6, almost nobody can beat you in lane. If you are confident of your abilities and have ignite up, you may try for a kill, otherwise, ask for a level 6 gank from your jungler.

It also helps to gank both bot and top lanes as Diana has a HUGE ganking potential

Obtaining Blue buff from your jungler helps, but is not necessary as you have more than enough mana regen to stay in lane

If your laning phase goes properly, the enemy AP champion will not have much farm and will be underleveled. You can use this to your advantage by killing off their Mage and ADC within 5 seconds, literally, or you can use your "E" to bring everyone together to give your team good aoe potential.

Please comment any suggestions you may have before criticizing my build, as I have had lots of success with this.