Rule number one is extremely simple and very easy to follow: be nice. It may sound like a lot to ask in a competitive community, but being nice is as simple as getting to know people instead of being a loner on the side of the room. Once people get to know you, if you have a blow up over a close match that you lost the onlookers will be more willing to understand and help console you. Let people know that you enjoyed the game you just played, or at least say, "Good games." Something I do if I lose horribly is say, "Thanks for the games." This allows me to be nice while recognizing that I totally blew it.
You do not need to coddle anyone. All you need to do is make sure that no one feels like they are being left out, or worse, ridiculed. Alienation is a quick way to lose new players who do not have a foothold in the community yet and no mentors or community friends. Give people a chance and you just may see an uptick in the number of people who consistently show up to your events and casual play gatherings. Start a conversation and get to know a few people. This is a simple rule of thumb when it comes to socializing. Yes, you will find a number of people you just do not care for. Avoid them. Be nice but do not subject yourself to their company if you do not like them very much. Just be civil and give them respect as players.
So you are on the path toward increased interaction with new players, and maybe even some existing community members you did not know very well previously. You are at risk of one potentially messy behavior and that is critiquing your fellow players. There are two simple questions to ask yourself before deciding to give someone critique with or without his or her permission. One, is the person in the right mindset to accept the critique? Two, do you know what you are talking about? If either or these questions results in a "NO", then please keep your thoughts to yourself. You will avoid negative interactions with your fellow players and you may also learn a thing or two about the game yourself if you let more experienced players take the lead on commentary and critique.
Now if the answer to both of the above questions is "YES" then you have another responsibility as the critic. In my opinion the avoidance of destructive criticism is essential to fostering positive community relations. Negative criticism that attacks the person's general demeanor or mindset is unhelpful and just plain mean in most cases. Constructive criticism that is specific, firm and conscientious will go a long way toward motivating the player who lost and also reinforcing his or her desire to be a part of your local community. Very few people will willingly subject themselves to a negative environment. The creation of a positive community will help lead to increased numbers of stronger players.
The final obstacle I see when it comes to establishing a positive community is how you deal with people who simply do not see it the way you do. These are the loudmouths, the hate spewers, the naysayers, and anyone else who is having fun but doing it at the expense of others. In order to deal with this I suggest reversing the strategy on alienation. Instead of interacting with these people in order to preserve their positive connection with the community, alienate them. Alienate them hard. It is mean, and you will have to get over that. Ignore people who are being dumb, raging a little too hard, or acting like know-it-alls when they really know nothing.
There you have it. This small primer will help get you in the right mindset to effectively grow and maintain your local fighting game community. Be nice, be courteous, and try to involve players in the community so that they feel invested and have a desire to return. Do the opposite for the idiots who sometimes show up in your community and push them out if they refuse to change their ways. There should be no room for negativity when the purpose of the community is to have fun and learn more about fighting games. Good luck and do not give up!