I have slight anterior pelvic tilt, it was much worse before. I'm currently doing some stretches and have to strengthen my abs because they're not so strong.
I thought I should have strong arch in the back when going down to DL, but that's what comes with neutral spine, right?
Work the glutes also, they are important on keeping your hip on correct alignment.
You don't need to arch too much in any point of the deadlift. The moment you start to alter the position and posture of the spine, the muscles react and different forces take effect. At this point the shear can come to your lower back area when you excessively arch your back on the way down. You lengthen your abs, deactivate your glutes, cause problems on the diaphragm and intra-abdominal pressure, and contract hugely on the spinal erectors. There's is lots of compensation.
Neutral spine means that there is no arching one way or another. Lumbar spine has a natural arch already. You want as stable and optimally aligned spine as possible. Mike Robertson explains the neutral spine better than I can, check this video out:http://www.youtube.com/watch?v=TwEJ7e_pm7o