NOTE: I am generally DM, so the following is based on what i *see*, not necessarily what I *do*.
First of all, becoming "partyy leader" can have a wide variety of meanings, and which meaning you prefer colors how you might go about it. Also, there is the question of whether you mean partyy leader (as in PCs) or group leader (as in players). in my experience, they tend to be the same person, and they tend to be the same person over and over again. It changes sometimes, based upon DM, setting, and group dynamics, but overall, if you play with the same group all the time, you generally see the same players (and their characters) take on the leadership role.
On to an answer that might actually be of interest and/or help to you: The best way to become party leader is to be decisive, but not overbearing. That means that when the party has listened to the old man in the bar and are talking about what to do next, you listen to everyone then make a decision. You don't want to just yell it out, but we all know how those "group decision making exercises" can bog down. So just make the decision. (By the way, if it turns out to be a bad one, guess who gets all the blame). This tactic should be used in any situation in which the game starts to bog down because of extraneous discussion. Everybody (including the DM) appreciates the player that can keep the game moving.
The other thing to do on a regular basis is act on behalf of the rest of the party. If someone is in trouble, heal them with your last potion. If someone has angered a tough NPC, go to bat for them. If two PCs are in an argument, resolve it in a way that is good for both characters. these kinds of things keep you on everyone's short list when they are looking for help and guidance.