How do I synchronize my iPod Touch with my laptop and desktop without having to erase it?

Ok. So I finally gave in and bought an iPod but am still a little illiterate when it comes to how to use it. I synchronized it with my desktop and have a mixture of songs that I've bought from iTunes and songs from other sources, and my photos, and my movies. I got everything I wanted to get onto my iPod and now have run into an issue. I travel quite a bit and have my laptop which has iTunes on it. I authorized my laptop but would like to be able to add music, movies, and photos that are on my laptop, but not my desktop, to my iPod. How do I do this because it asks me to erase my iPod. Isn't there a way to be able to use my iPod with my desktop and my laptop, adding new content to my iPod from either the desktop or laptop without having to erase the iPod?