Nearly everything is electronic with me. I organize on the computer using one folder for the trip, then subfolders for each destination. I organize my bookmarks/favorites the same way. Every time I come across a webpage that is relevant - including threads here on the forums - I add it to the relevant favorites subfolder. When I find a useful pdf, it's downloaded to the subfolder. Later on, I can cull through the material and delete as needed. The Snipping Tool is so useful. I use it to create word docs, even simple ones like how to get from the airport to the hotel. I use it to copy from online articles (especially wiki) to word docs. I working with lists and tables is much easier on excel than word. I have one workbook, with multiple tabs (calendar, hotel info, sights info, packing list).
I start a word doc for each destination. Along with information I gather, I also put in my notes and thoughts. As I travel, I try to keep a journal (often I end up doing it at home later) and a detailed list of my spending. Keeping track of spending helps me stay within a budget (and know when I can splurge), and it's even more useful for planning future trips.
I use some guidebooks in planning too. I will take 1 or 2 with me, but if I only want a few pages from it, I scan them and add to the subfolders.
I travel with a notebook computer. I transfer all my files and favorites before I leave. I also back up my photos daily to it. I also have an iPod Touch (basically an iPhone without the phone) with all my files on it. I use Dropbox to back-up and transfer the files to the Touch.
The only things I print out to take with me are self-guided walking tours and maps, tickets (train, plane, sights).