All I did was deadlift more and more.
First of all though my max deadlift never went anywhere until I learned the correct technique. Your back can do the bulk of the lift for you now but if you are ever going to go past this point you need to fix your form and get the legs into the movement more.
I used to deadlift every Friday without fail. All I did on that day was deadlifts. Working up to 2 rep max or 1 rep. usually did about 8 sets total and then left the gym
That worked for a good while and got my deadlift way past 405lbs.
Other than that, things I do now to get it up are;
use straps, you will get more reps and a stronger deadlift, at the sake of losing a little bit of a grip workout.
Rack deadlifts - Allows you to use more weight, speaks for itself really.
Strengthen the hamstrings, doing romanian deadlifts, good mornings etc.
As well as all this, dont be afraid to add more weight. Some people stick to a weight for too long because they dont want to fail a bigger weight in front of their friends.
Right now My advice for you is to practice with lighter weight until you feel your form is solid. Not looking for perfection here but it should be safe and your back should not round to the point where its dangerous.