Make sure to make the most of your Q in teamfights, i always try to hit as many people as possible to get the most healing.
Try to focus one enemy in teamfights so you can start applying 5 stacks to all the enemies so you can multi-ult and clean up a fight.
Build tank NOT AD. Darius has high damage even when building tank, and this also allows you to benefit more from Q heals.
Don't get over confident in lane, even when you know you have a good match up. I have seen many cocky darius feed early and end up losing the game because of it.