A dressing is basically an emulsion. Without getting too heavily into it (you can dive in depth at the links below) its essentially how to make the dressing thicken and bring them together so they don't separate. This involves having an emulsifier and bringing the ingredients together gradually (I use a mini hand blender/food processor and open the top put a little oil in, close, blend, open, etc) or if I am using a jar to shake the dressing I do the same process but shake it 10 - 15 times in between adding the oil. The second link below goes through a whole 101 in salad dressing, do the section on partial emulsions and then full emulsions. (Sample of a recipe that simply describes the emulsion process in a recipe: http://www.foodnetwork.com/recipes/alton-brown/veni-vedi-vinaigrette-recipe/index.html)
Finally, there are alternatives. Don't have oil on hand? You can use egg yolks (commonly used in caesar salad dressing). Want to take an oily dressing and make it creamy without adding mayonnaise? Add tofu and put it in the blender or use sour cream (fat free if that is what you are looking for) or even better plain yogurt.
If you taste the dressing and it is missing something, often a dash of salt will help bring it all together. Another test for flavor if something seems missing is to run through the flavor sensations, for me the salad dressing is an art of finding the right balance between tangy, sweet, citrus, nutty, earthy, etc).
Keep in mind that a standard proportion of vinegar to oil for dressing is 1 part vinegar to 3-4 parts oil
http://www.bonappetit.com/test-kitchen/cooking-tips/article/become-a-better-cook-4
For more on the science and a much deeper dive into these principles please read the following: http://debbrunson.com/2012/02/05/sauces-dressings-101-permanent-emulsions/
Simple Traditional Dressing (from Italy)
Olive oil
Lemon juice
Dash of salt
(Used with arugula salad with lightly roasted cherry tomatoes and parmesan)
Orange Dressing
Orange marmalade, rice wine vinegar and sesame seed oil
Use reasonable portions, mix to taste or use above guidance (1 part vinegar to 3 - 4 parts oil)
Honey Lime Dressing
I use this base for an Asian flavored salad or a Mexican salad.
Lime Juice
Honey
Splash of balsamic (can do it with balsamic or keep it more tangy with just the lime juice)
Olive oil
Additional seasoning: ginger for an Asian based flavor, cumin and cayanne or pepper flakes for a mexican bent
Use reasonable portions, mix to taste or use above guidance (1 part vinegar to 3 - 4 parts oil)
http://feetinkitchen.blogspot.com/2013/10/cilantro-green-garlic-vinegrette.html
