I think my favorite part comes before I even start a lot of writing. I enjoy thinking about the main characters, what do they like, what are they like, what are their motives, etc etc. I just really enjoy watching them take shape.

As for the other, I'm lucky to have a couple good friends who read and edit for me and I usually show them the wrongness and see if they can help me find it. If not, then if at all possible, I set it aside, work on something else and come back to it a few days/weeks later with fresh eyes. I can usually find it then