We never realized how much harder it is to write a sequel. When you’re writing book 1, you can make things up as you go, but when you get to book 2, you have to make sure you don’t contradict anything from book 1, tell the story for book 2, and also drop hints and pieces for book 3. So many moving parts! Thankfully we have huge spreadsheets full of information so we don’t have to rely on just our feeble memories. And there’s always the search function in Scrivener.
We also use the spreadsheets for the layouts of buildings/apartments/offices, so we can visualize what our characters are doing. Each new setting needs a new diagram.
We have a page for forum characters, which we’ve found very useful. If we need a new character for a short story or Book 2, we have a pool of ready-made people to choose from. (We had a great time creating over one hundred forum characters in the first place.)
Perhaps the richest spreadsheet is the links page. It’s where we keep links from our internet explorations. Here’s an interesting one on how to disappear completely: http://www.wikihow.com/Disappear-Completely.