My wife and I were both constantly working on planning aspects of our wedding and reception in Vegas; often at the same time of the day and from different locations. This meant using papers or planning books would not have worked very well for us as we'd have to keep giving bits and pieces back and forth whenever there was overlap or one needing to know where the other was at. Additionally, papers can get lost or forgotten when you most need them.
The solution to this issue for us was Google Docs. I'm a Google/big-brother conspiracy theorist as much as anyone and try to keep very little personal data accessible to them, but in this case it really made things incredibly easy for us. We both have gmail accounts so I created a Google Docs spreadsheet and shared it with her gmail account as an editor. Now we have one web-based spreadsheet that either one of us could edit at any time from any location, whether it be computer at home, laptop, phone or tablet. I could make notes in it while talking to vendors using my phone, clean them up later from my regular computer. We could add information simultaneously and it wouldn't affect what the other was doing. Having it on our phones meant we would have the planning info accessible to us everywhere.