Before I start a scene or chapter I've generally got the whole thing 'shot' in my head. I know what angles I need and where everyone's standing and I also know what's going to be said and how, just not in exact words. I cross the dialogue bridge when I get to it. It's like I can already see the results, I'm just remaking it on the screen.
The problem with this approach (and there are lots) is that my panels aren't very creative. I've been raised on film mostly, so when I generate the comic in my head I'm 'viewing' it through the camera lens I'm accustomed to, which in turn means that when I draw everything comes out as if it were being seen on a screen.
Sometimes things fall into place and I'm able to bring a little uniqueness to the page, like with this page and this one.
I'm also working on this one right now is quite fun with angles and stuff.

I also follow the 180°Rule when it comes to angles and dialogue which helps massively with flow.