As other people have said, don't get downhearted if people leave. Just let them be and keep recruiting! Also don't be surprised if people join and become inactive or never even log in again. Make sure you set a limit of how many days a person should be inactive for before kicking them so your FC isn't full of inactive players.

Make sure you have FC chest settings applied so nobody can ninja your gil or items.

Help people and be talkative! You don't have to speak all of the time but people are more likely to stay if you communicate with them and make them feel welcome. Say hello to members when they log in and offer help if they need it. Also tell members they are free to invite others, you may even get more members.

On the contrary, have your rules set and stick to them. If you're running a serious guild sometimes you will have to make decisions not everyone will like. Remember that you can't please everyone.

Good luck with your FC!