I just recently transitioned to sumo. A fantastic lifter from NFLD, Canada (he actually won best lifter at IPF Master Worlds, not just his weight class, but best lifter overall) and his crew suggested I try it out.
I've found myself agreeing a lot with what Fazc and others have said in this thread. Good deadlifters deadlift a LOT. In fact, good raw lifters hit their lifts very very often, drilling PERFECT form in every single time. I had not used anything more than what I considered 90% of my sumo max for the 3 months leading up to my meet and pulled a 2.5kg meet PB with plenty in the tank (so a PB AND it was with a different technique).
Lower your training max to something that you can pull with PERFECT technique, and then start working away at lots of volume of that. Preferably, though, lots of sets and not too many reps. anything between 65 and 80% of this new PERFECT max will be great for you. As things start to feel more natural and you can move up, do so. As Facz just said, perfect practice makes perfect. You don't need assistance work, you need to drill this movement pattern in.