Things that I've learned that help my guys a lot are as follows.
1: Play around a bit with your stance. Watch vids of lifters with somewhat similar proportions to yourself. Once you get that straightened out...
2: Knees out hard.
3: To be somewhat crude, try to put your balls on the bar. That goes in line with #2, knees out hard. This will build tremendous tightness in your hips.
4: Low back arched hard.
5: Chest high. Not head high, but chest high.
6: Hard one to explain, but you have to sort of flex your hips to break the bar off the ground. If you're trying to put your balls on the bar, and your knees are out very hard, you'll find that it's not so easy to reach the bar. If you can hold this tightness it can result in very good explosiveness off the bottom, which is normally the weakest part of a (raw) sumo pull.
I'm not a good conventional deadlifter, and less so for sumo, but incorporating some of the above things got me a 545 pull last time I tried, and I believe I had another 10-20 or so in the tank.