Staff members are chosen using a set of criteria. For some positions, there are applications. By applying, you automatically indicate you are interested in that position. For other positions, such as Helper, the Server Managers converse with the Staff Managers and Sawine who would make the determination on promoting a player to Helper.
There are a number of things that would cause your application to be passed over, which include but are not limited to: Asking staff members to review your application, constantly stating in-game or on the forums/wall that you have applied, previous records of rule-breaking, and/or applying for every staff position available.
The best way to obtain a staff position is to apply for the one position you are most interested in, help other players out in-game, and be patient.