I don't make my own dressing, but there are a ton of low carb at the grocery store. Even lite dressings are low carb--I think Newman's only has like 3 grams per serving.
If you prefer to make your own, I guess you should stick to vinegar based dressings. I'm guessing what you're making is some kind of ranch or thousand island? Pretty much any oil/vinegar based dressing will be low carb.