Agreed on a lot of what others have said here. I actually don't edit as I go at all, the one exception being if I've taken a long break from writing a longer fic, then I'll usually reread/edit what I've started with to find my voice again, otherwise, I write everything, or most everything out, and then go back and edit. For me, it's a lot of trusting my gut/instincts. If there is a section/sentence that is reading weirdly, if I feel like a characterization isn't quite coming through, etc. I'm probably right. Very often the things I've felt wibbly on but didn't feel like dealing with so hoped they were good enough were things my betas ended up pointing out.

For me, the biggest thing is finding people to look over my work. If it's longer than a one-shot, I'd like at least two, preferably three eyes on it. These should be people who you click with, who give the kind of feedback you're looking for. For me, that means squee included in the feedback, but also more than just squee and basic SpaG. Like magpie said, I find people who are good writers themselves and/or voracious readers (of more than just fanfic) make better betas, but not everybody is cut out for giving helpful feedback.

I'd also say, don't be so tied to anything that you're unwilling to part with it/cut it/change it if it's better for the story. There have been a number of times I've had to cut down or completely eliminate/change scenes/conversations etc that I loved and had put a lot of time into writing because they weren't working for the story. One thing I see a lot of with longer fics is a lot of unnecessary scenes that don't actually push the story forward, and it ends up really dragging down the pacing. You might love a particular conversation, but if it's not serving the central narrative, it might be better off being cut. And you can always rework them into little bonus scenes/codas if you really can't part with them. :D