In today's modern world, though, I'm more likely to have my smartphone with me than a pen. Thankfully, there's an app for that. You can turn your phone into a virtual napkin.
I personally have a google document set up to be my virtual notepad via a Google Drive app. This method allows me to draft while lying in bed unable to sleep, walking around my campus, waiting at my son's baseball practice, or sitting in a boring meeting. Lately, I've been able to put several thousand first-draft words on my phone at a time. I later transfer them to my laptop. This method works well for crafting new material. It's not going to help when I'm in editing mode, though, as a whole manuscript would be unwieldy on the phone.
I also have a google document called Story Ideas. It started out as a text file on my phone, but I uploaded it to Google Drive so I could access it from any computer. I use this document to store my shiny new ideas, sorted by date. I have dreams of sitting down with an agent someday and combing through the list for the best one to use in my next project.
Anyway, what do you use to take notes on the go? Pen and paper? The back of your hand? Or have you found a tech-savvy solution?