Darius's Laning is incredibly strong. His Bleed offer's a free harrass. Each one of his abilities bring a Stack of Bleed to the enemy, Bleed could be stacked up to 6 consecutive times. Plays I love to do as darius at top lane when the enemy top laner is low in HP is,

Flash+W+Q+E+Ignite. By then you should have a fair amount of stacks on the enemy, and ignite would just secure the kill for you aswell with your Bleed.

Answer is simple, whenever you guys farm, you Q while farming. Make sure your Q is in distance to them. Once you hit level 2 be sure to get your W, then whenever the enemy tries to farm Q+W. Your W gives you tad bit of extra range with his auto attacks. It also slow's the enemies down. while the enemy is slowed be sure to auto attack them more and more, keep in mind. The more bleed stacks. The more they will be set back in HP and will be zoned and afraid of you.

In teamfight's, be sure to combo there AD carry. Such as W+E+Q+R. Also! be sure to last hit the enemies with your ult. As your ult gets a Cooldown Reset when you Kill an enemy with it. However it's tough to kill a whole team with your whole considering your ult costs 100+ mana. and if you burn your Q+W+E it'll be tough to have that mana still within your capacity.

There are tons of places an Enemy jungler could gank from at top lane, Best thing to do is ward,Freeze lane, and do your best to Zone. I'll post a Screenshot of potential Gank Zone's from Top lane.