DS was a big baby (10lb 5oz) and I fell fortunate that everything was done properly.
Fundal height measurements were ahead so I had a growth scan at 35 weeks where they predicted that he would be about 10 and a half pounds and referred me to the consultant to discuss options (elective c-section vs. trial of labour). When my waters broke he still hadn't engaged so I was treated as high risk throughout labour. They did put me on Syntocinon to speed dilation but always with one eye on the possibility of a c-section (actually, the syntocinon was the one thing I wasn't particularly happy about). When I got to second stage I was introduced to the team who'd be doing my c-section if we didn't progress (they didn't quite phrase it that way, but that was the general idea) and when pushing wasn't moving him down very far or very fast they were happy to call the c-section.
Picking up the size either before or early in labour, combined with taking it seriously without panicing the mother or pushing her into a c-section, seems to be the key thing so far as I can see.