Using arms and legs:

You can stick out your arms or legs in order to let your presences known to nearby drivers who are getting to close to you. Its hard to see a cyclist because we are so narrow, drivers are usually looking for wide objects and when you extend either your arm or legs toward the drivers view they are more likely to see you.
You can also use your arm to tap or put your hand on the car, that will definitely get the drivers attention, usually I do this when there’s no time and I need that driver to recognize I’m there or I’m toast. Word of advice… don’t hit that car side, many drivers will not like it and react to you aggressively and you will end up getting hit or scared shitless. Sometimes when you want to get someone’s attention you tap them in the shoulder, you don’t smack them in the head. Unless you really have to. LOL.
Or if you don’t have enough time to signal your presence to car you can use your hands as a buffer to keep the car from knocking you off your bike. Push your hands against the car to guide yourself away from the car. The mirrors work best for this. This can work pretty good when you are getting hooked by a driver on an intersection. In this case go with car’s turn, and use your hands if it’s to close to you.

Obviously if you are on a fixed gear using a leg is out of the option. That’s why I enjoy my road bike, because a leg is bigger and thicker than an arm, so it’s easier to spot. And I can still be able turn my bike pretty good while I do so.