When I make fat free salad dressing I add a tsp of either Thick & Thin Not Starch, or xanthan gum. This makes a nice thick texture, and the dressing sticks to the salad better. It also adds healthy fiber to my diet.
I only use this when I have other fat in the meal to make sure I can absorb all the nutrients in the salad. I like to add Goat Feta or avocado to my salads. That's when I whip up a fat free dressing to save calories.