I don't have a hard and fast recipe (read - I don't measure) and it always comes out fine. The blender* emulsifies it beautifully and with no effort on my part, which is the real secret. I still love the mayo/yogurt based Mexican-ish Caesar dressing, but for everyday I tend to make this one, or one that is basically identical but uses lemon juice instead of vinegar and has a smidge of anchovy paste and a couple cloves of garlic.
Super simple balsamic vinaigrette (makes enough for a couple salads, scale as necessary)
1/4 to 1/2 cup olive oil (adjust to your preference. I'm somewhere in between)
1/4 cup balsamic vinegar
Salt and pepper, to taste (about 1/2 tsp salt, a few grinds of pepper)
Optional (but highly recommended) add ins:
Toss in a clove or two of garlic
Add a few leaves of basil
A heaping teaspoon of good mustard
It holds up for a few days in the fridge although it might need to get shaken up before you use it.
* You guys, I know I mention the tiny blender all the time. It's just that when I registered for it I had no idea how much I would use it and now I'm a tiny blender evangelist. I have no stock in Tribest, I swear. I think any small blender would serve exactly the same function. Just make sure you get one that has lids for the blending cups and you'll be set. I haven't purchased salad dressing in years and I don't mean that in a superior way, it's literally just that making it is easier than remembering to buy it.