I've been playing with LeBlanc for a while and I can tell that her only enemy is Kassadin. If you want to succeed you should be able to decide when to be agressive or defensive and what to do in any situation. First, you should try last-hitting at level 1 and 2, just ignore your enemy and don't keep the distance. At level 3, use your Q and W together for the highest damage, or if you have an enemy that likes to keep distance like Xerath or Lux, just jump close to him and give him a Q, then immediately teleport back with your W. When you reach level 4 you can call your jungler for a gank as you can root your enemy for a short while. Enemies usually use flash in these situations and start to be defensive so you need to bait them if you can't handle them alone, otherwhise just use your burst and try to lock down your enemy, so you can give him 2 Q and that should be enough to send them back to the base. If they start stacking Magic resist it means they are lack of damage so you should only harass them a bit more than before, but if they are still a pain for you, buy the Void staff or the Abyssal Scepter before Rabadon's Deathcap and you will be fine. After you win mid lane (killed your enemy at mid at least 2-3 times and he's at least 1 level below you) start ganking. you can Go with your jungler to be sure to get a kill or move alone and send them to the base. With the Rod of Ages you should have enough mana after early gamebut always ask for blue if your jungler don't want it so hard, because the cooldown reduction is awesome for LeBlanc. (Sometimes I even consider to buy a Morello's Evil Tome, after Rod of Ages but it's not necessary.)In late game your job is very important: 90% of the players think that LeBlanc is weak in late game, I admit that she's not the best in late game but her killing power is still enough to instantly kill a squishy enemy and to damage the others, so in teamfights you have to go for the AD or AP carry (it depends on who's the bigger threat) and kill him immediately, then if the others focus you, try to get away until your skills are off cooldown and continue the fight, if they start running from you just harass them until you have your jump and chains and get them too, but always co-operate with your teammates, even if you're a killing machine you shouldn't get a stat like 30/2/0, give some kills to your mates or kill the enemy if it's possible only when you have low farm and desperately need that gold. For the items, Rod of Ages is very important in early game, the Void staff and Rabadon's Deathcap are must to have as well, but if you feel like you don't need bonus HP and a slow on your enemies you can switch Rylai's or Abyssal Scepter to a Morello's Evil Tome or for Zhonya's Hourglass if you face with 3 or more AD enemies and they all focus you.

It is very important to know how to and when to use each ability. Your full damage burst is Q R W E Q (if the your E locked the enemy down). You can harass with W Q E and keep your ulti, or W Q R E for more damage. if you want to prevent enemies from running use E and if they flash jump after them. you can use this with double chains if you gank bot lane for example. When you're low on HP and recall under turret but an enemy tries to get you, use chains on him and double jump towards your base. This sometimes not only gets you safe but you may even get a kill or force your enemy to use a heal or flash.

The magic pen marks give you some bonus damage in early and helps you if the enemy has some magic resist.
The mana regen seals and masteries makes you possible to start without Doran's ring and to end your build 4-5 minutes earlier.
The ability power glyphs are good for higher overall damage, but the difference can be seen quite well only in early game.
The Ability power Quintessences are even higher starting bonus, with these I start with 30 AP without items.
The masteries are the normal for any AP carry with some mana regen, longer buffs, some magic pen and AP and overall damage increase.

I wrote this Guide, because my experiences with LeBlanc are quite good, I lost mid lane only 3 times, always because of a Kassadin, so watch for them. Even a Sion or Mordekaiser can be beaten on mid with some strategy and the right usage of your skills. Good luck to all of you who read and try my guide, and remember that it's a team game so co-operate with the others.