So you're running a game at a con, or at the local store, or for a G+ hangout. It's only going to be one session, and at least some of the people may not have played (or even heard of) the system before. What can you do to make the game run more smoothly and be more fun and satisfying for the players? Here are some good ideas:
Sell the game - don't assume everyone knows what the game is about. Give people the elevator pitch at the beginning, so they know why they should be interested. (An elevator pitch is a sales pitch that can be delivered on an elevator ride down to the first floor of a decent-sized office building; half a minute or so.) Also give them an idea of what they'll be expected to do as players. For example, if you're playing Monsterhearts, tell them that they're teenage monsters in high school, that they're going to be horrible to each other, that they're going to have sex with each other, and that they should aim themselves at each other's characters as aggressively as possible.
Start with character concepts - tell the players the types of characters they'll need to create and walk them through the process. Focus on themes and archetypes instead of rules or mechanics. For example, InSpectres says to create "normal people" who used to do other jobs before they became monster hunters. If a game has playbooks, point them to the one- or two-paragraph summaries of each to see which speaks to them. In most of the Apocalypse Engine games these are on the playbooks themselves, so it's easy; in Dungeon World there are about four pages of one-paragraph blurbs on each class early in the book.