A friend was needing vinaigrette ideas, so I wrote this out for her, and as I got into it I realized it's totally blog material. So I thought I'd save it in case I need to share it again!
A traditional vinaigrette is: 3 parts oil to 1 part vinegar (and some mustard, salt and pepper). That means that if you want to make 2 cups of dressing, put in 1 1/2 cups of oil and 1/2 cup of vinegar. But the 3:1 ratio works at any quantity. Apple cider or white vinegar is best as a staple.
To pick your oil: Olive and canola oil are the two most popular for dressings. Corn oil is also fine, especially if you'll be cooking. If you want to use a flavored oil, stick to 1/3 flavored oil and the rest
With that quantity in mind, add about 1/2 tbs of mustard, any kind EXCEPT yellow, dijon is preferred--this is to help the oil and vinegar stay together (called emulsifying) and is thus an integral part of the dressing, so if you're going to omit it for any reason, you may want to include something else to help it emulsify--garlic helps, as does honey and egg yolk.
Add any seasonings you want, in whatever quantities you desire, though if you use a lot of dried seasonings you may need to add a little extra water for consistency. I'll list a few favorite combinations below. Be sure to add a little salt and a lot of pepper, and burn a few calories shaking very well.
This will stay in the fridge for ages, and is safe in the counter for a few days. Typically, store-bought dressings have hydrogenated oils (trans fats!) in them so they stay liquid even in the fridge. Using non-hydrogenated oils, you will need to let this sit out at room temp for 5-15 minutes before shaking and pouring, so the oils can melt.
If you ever feel it needs to be sweeter, try adding a little apple juice
or apple sauce; failing that, any fruit juice or puree will do, but
will be more likely to change the flavor.
Italian dressing: minced or powdered garlic & onion, red pepper flakes, basil, oregano, (I like adding parsley and marjoram and thyme too, but they are less necessary).
Ranch: Garlic, sage, oregano, pepper, and if you really want you can add a little buttermilk, but even without you get a kind of ranchy-flavor.
Sesame Soy: Substitute up to 1/3 of the oil (half cup in our example, though 1/4 cu would probably suffice) with sesame oil; add a few dashes of soy sauce, plus some dried ginger, pepper, garlic, and cumin. For a more gingery dressing, use fresh ginger.
Taco Salad dressing: Put in a couple dashes of tapatio or chipotle tabasco sauce; add cumin, chili pepper, garlic, onion powder, cilantro (dried or fresh), and red pepper flakes.
Strawberry/raspberry/fruit: Add a dollop (1 tbs or less) of your favorite jam. Or, 1/4 of fruit puree for our two-cup recipe (of course then it will be more than 2 cups ;).
Fruit Balsamic: replace 1/3-1/2 of the vinegar with balsamic vinegar, and add jam/puree as above.