Depending on how good your etching is, you can route traces under 1206 and 0805 resistors/caps, and of course, between the pins of a DIP IC or header. Sometimes using a ground plane makes routing easier, so long as you get the isolation right, so its not too close (I used 24 mil spacing I think..). Generally you place things where you *want* them to be, try to route, then move some things around when you hit a brick wall. Connectors around the outside of course, but anything else can be moved.