Acid + Oil + Spice = Marinade
The base of marinades is very simple: an acid, an oil, and spice. The acid works to break down proteins thereby making the meat tender, the oil lubricates the protein and makes the meat juicier, and spice of course flavors the meat. Many marinades also have an emulsifier which is usually an agent that has both a hydrophobic (water averting) and hydrophilic (water loving) side. The emulsifier can add flavor (for example, Dijon mustard), but mostly it just binds the acid--which is made mostly of water--to the oil.
Once you know this simple formula, the possibilities are endless.
What ratio do I use, you ask. Well, it depends on what you are cooking. If you are using a tougher meat (for example an eye round or a pot roast) increase the amount of acid as compared to an oil. Also, allow the meat to marinade for a longer period of time. Chicken should marinate less unless you want to eat mush (although tonight's recipe requires that the chicken marinates for 24 hours). Fish has such a delicate texture that unless you actually want to cook the flesh in acid (sometimes you do, for example when you make ceviche), do not marinade for more than 30 minutes.
The most common acid is vinegar, however you can also use citrus (lemon or lime with fish or chicken is delicious), wine, buttermilk, yogurt....you get the idea.
I grew up in upstate New York and one of the treats we craved the most was Brooks chicken...slightly salty vinegary New York style barbeque cooked over a pit. For all of you southerners, we know how to bbq in the North as well. The Brooks recipe is actually based off of a recipe developed at Cornell University by Dr. Robert Baker who, according to the Cornell website, came up with Chicken nuggets as well.
Incidentally, Saturday is National Kitchen Klutz day. I'm not making this up folks. I read it on the internet, so therefore it must be true. You can even send a greeting card to your favorite Kitchen Klutz! Let the celebrations begin!