"looking up when jumping with back straight"
Im no expert, but I have watched a few bocking videos and I'm not TOO bad myself, in terms of getting good height...
But I've heard that to get extra height you need to (obviously) use your arms, but also, if you bend your body at the waist and then pop it back up you gain some height, once I perfected this technique I could bottom out my bocks (im 60kg and they are 70-90 Pr's) and get some seriouse height.
but for a begginner, i dont reccommend this... but if you watch some pro bockers they do do this...
and ofcourse i could be completely wrong