Once you have the basics down and start to hit sticking points in your deadlift, it's important to use special exercises to help build the top and bottom of the lift.
Deadlift From Blocks
To work the lockout, block pulls are king. These are better than rack pulls since the bar flexes just like it would off the floor.
If you don't have blocks, you can stack up plates or steps. There are just too many ways to cheat when pulling off pins. And when it comes down to it, you can't really mimic the proper deadlift position unless you're pulling off blocks. It just transfers better.
For bottom-end strength and floor speed, deficit deadlifts are awesome. Standing on a plate will make the start much harder and help build the bottom of a deadlift.
This is extremely important if you have trouble moving weights off the floor. No need to go crazy with the height on these. A few inches is fine for deficit pulls and can lead to huge gains.
When To Do These Exercises
These supplemental moves should be done after the primary deadlift work sets. Generally, sets of 5-8 reps work best, but heavier triples and singles can be done on occasion in place of traditional deadlifts.