Paint sanding question

I'm not sure if I'm in the right sub-forum, but here goes:

I haven't prepped a ride for painting in a few years so I forgot something. How do you keep from cutting through an edge while sanding a fresh coat of paint? By edge I mean something like a character line that runs the length of the vehicle. My 73 Dart has several of those. If I'm not careful it's easy to sand down to bare metal.along that "peak". I think there's a technique but don't remember what it is. Thanks.