I highly recommend packing dental floss for your extended travels or backpacking. Get the largest roll in the smallest package that you can find — and like. (If you don’t like the feel and use of your floss you won’t use it.)
Flossing removes the plaque from your teeth, keeping you and your teeth and gums healthier, but there’s more.
You can use it to scrape your tongue. I know, some toothbrushes have a tongue scraper on them, but you should be taking a travel toothbrush or taking the smallest, lightest toothbrush you can find. Plus, the supply and style of toothbrushes vary between countries and you may travel longer than is good for one toothbrush.
I have heard of people using floss in lieu of string as well.
NOTE: I wrote this May 2012, before I knew about Cocofloss. Now, although I wish the Cocofloss container was smaller, I no longer recommend you buy floss for a package size. See my Cocofloss post.