Diana hits hard late game and has a lot of jukes and tricks up her sleeve when built correctly and used at the right time. She is all about timing.
Until 6 Diana's only excape is her flash if you take that. But it is best to make sure you are CSing and not showing that you may be afraid of the laner. Better to be a bit aggressive while staying hidden behind minions and getting your CS than staying so far back you fall behind.
Having to CS under turret is always an option that even I have to resort to depending on who i am facing and how bad the game is going.
Never just use your Ult, it will go on straight cooldown for 20 seconds, which doesn't seem long unless your in a team fight or a 1v1 that could turn ugly fast. Always try landing a q then ult so you can ult a second time.

Pros
Can poke from a distance with her Q
Her W allows sustain with the shield that is given from it
Quick cooldown on ult
Q allows for ult to be used twice
late game her Q CDR is so low your ult is always up anyways
Once level 6 she becomes scary
Good initiator
Good at split pushing
Nukes most champs down including her own counters (mwahahahah)

Cons
Depends on skill shots
Mana Hungry
Pretty weak until level 6
Melee, so hard to face ranged mid laners early on

Ignite and Flash are probably the most useful when building Diana this way, she can be pretty bursty but in early levels she needs that extra "umph" to get a kill. That is where Ignite comes in handy because if they try to leave you can E (pulling them in) and ignite them along with getting one or two more hits in. Also if they are already low it is nice to ignite and still kill them with skills, many say it would be a waste but if you get the kill I just say that it is insurance in case their flash is still up.
Flash is very handy because early game Diana doesn't have the speed to get away so flash helps get you our of sticks situations. Once you have boots of mobility it helps you catch up and get a kill by flashing in and still running then q/r/r combo. (Or any other combo that you need or want to use)

Is pretty important for every champion. With Diana she needs the gold fast to start building those expensive items. I have been told by many of my friends that I got HAM with Diana and tend to have pretty low CS, my average being 200-250 (now that I have focused on practicing ^-^.)
If you are unable to get in and personal with the minions use Q to poke and last hit a group of them. Late game your Q and passive will wipe two waves worth of minions in no time. Usually I will W for the area attack/ Q to hit all or most of the minions then leave my passive to do the rest and clear the waves. Also using your e to bring them all in close will help with wiping them out with your passive.
So if you can not cs early game late game is easy and quick work for Diana, one of the main reasons she is good for split pushing. Able to wipe out minion waves and push up to tower with a small army behind her in full health because she wiped out the waves so fast.
If team says go for it, take on a lane that is being pushed to far close to your turrets and push that, it may or may not catch the enemies attention, if so quickly move to another lane and push that. Eventually it will allow you to get a tower or two. The power of boots of mobility and quick farming.

This is the end of my updated version of my first guide, mostly changes to items and runes since over time players change their style to fit what is going on in the game. Thank you for you time and even willing to look at this build. Have fun with Diana and show them she ain't no one to ignore!