1. Help whenever possible, but only when asked. This coincides with one of our Ten Commandments of Gaming, "encourage new players and invite them back." More experienced players should create an environment of openness to players who are learning. Much like a good college professor or a mentor, its about being a teacher/student. Don't let this go to your head, because this is only a temporary situation. A good teacher can have a student learning on their own and instill a sense of confidence in their students. Sometimes it may get to be a hassle to help new players learn a new skill, but it is so very worth it when a player finally "gets it". Answer questions for a new player, but encourage him/her to find answers on their own. Point them in the right direction.
2. It may sound tempting to know everything about a game system, but it isn't worth it. Don't bother. Start with the basics. Focus on learning the core mechanics and the other nuances of your game usually just fall into place. Let your knowledge grow naturally. There is no need to rush to catch up to the other players. It may even take a few years for this to come to fruition, but again, it's so worth it.
3. Find common ground. Experienced players should remember when they were first learning a game and what it was like for themselves. New players can aspire to find the characters that are right for them, and the methods in which they like to play. All players can swap war stories about their favorite moments. Every player, whether old or new, has a great opportunity to make friends that you may not otherwise have had a chance to meet.
*Bonus XP: All of this ties into one easy statement: Drop the pretense of superiority! There is no need for such a counter-productive mentality. It's this common perception in the public eye that scares new players off. Do some real good for your favorite pastime and your friends, and just do a reality check. Help, learn, and don't sweat the small things.