I am a fan of using multiple monitors when I can, but when you are out and about a lot this becomes difficult. You can see on Amazon that there are some pretty portable USB monitors which I was thinking about giving a try. That is when I heard on a podcast about turning your iPad into your second screen.
I had my doubts, but it works! The app is called Duet Display. You need to download an application from their website on your Mac, and also download an app from the App Store on your iPad (or iPhone if you wish) which costs $15. Restart your computer, plug your iOS device into your laptop via USB, and off you go. It works exactly the same as if you attached another monitor via your display port. So now I have a second screen for my laptop which I usually carry in my work bag anyway.