You can mark houses visited/not-visited -- were they not visited because nobody was home? Scary dog? (literally has a spot for "scary dog, can't go to door") Do the same people still live there? Is the house on the market? Does the house have a foreclosure notice? It highlights if someone in the house has turned 18 since the last election -- do they still live there? Are they registered to vote?
Eyebrows McGee offers some first-hand insights on how the NGP VAN database works for candidates and and campaign workers.