I have some design diagrams only on paper. Scanning them to bitmaps is easy, but I've had no luck getting useful vectors out of them. I've tried vectorizers in programs like gimp, and a few online services. Generally, I end up with enormous numbers of spurious vectors (from dust, dotted lines, text on the diagram, slightly variations in scanning contrast, etc).
What tools and/or techniques can I use to get a more useful vector result, that I can then modify in a normal CAD tool without spending absurd amounts of time cleaning it all up first?