With the help of GpsMid (http://gpsmid.sf.net), a J2ME Midlet to display vector maps on a Java-enabled mobile phone, I've been adding more detail to the areas that I've been mapping. Although I don't yet have a GPS receiver, GpsMid will interface with one via Bluetooth and I intend to take full advantage of that fact the next time I'm in Mansfield and I can nab my brother's. In the meantime I'm making extensive use of GpsMid's Save Waypoint feature to mark significant places by hand which can then be more accurately positioned when I have actual GPS data to align things.
All of the back streets I regularly walk on my way to and from work are now mapped, mostly by tracing over the NPE, some by tracing someone else's GPS data. I've started to venture further afield, taking different routes home and pounding different streets when taking the kids to the park or nipping down to the shops so I can add more streets and more details. There are still some significant gaps where I don't know enough about the layout of the streets to reliably trace the NPE, but again I'll cover those when I'm GPS-enabled.
My eldest son likes to see the map on Daddy's phone and seems to understand that we're helping to draw it. My wife and people in the office are probably sick to death of me banging on about it and constantly polling them (and even complete strangers in the street!) for details, always mindful of the need to draw on their own knowledge, not that gained from Google Maps or similar.