The best thing to do is just write good posts/threads and be patient. Your gil count will rise
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The amount of gil given for posts/threads used to be higher, but some people spamming just to get gil resulted in it being reduced. Shame, but the minority doing the wrong thing ruined things for everyone.
There is one idea that all members can think about to earn some extra gil. How about some forum competitions for all members with gil for the prize?
Things like a best avatar contest perhaps, or a sort of peoples choice awards for best poster, most populour member, most helpful member ect. Something to think about.