Before looking at "how" to earn that promotion, you have to examine the question of why you want it. In fact, it's usually the ones who don't care about the rank and who go that extra mile anyway who make the best officers. On the other hand, having officially be tabbed as an officer makes you a more visible leader, not only within the guild but to other guilds and players on the server. And being an officer is a validation of your leadership abilities as well.
It's OK to want to be an officer, but make sure it's for the right reasons. Actively lobbying for the role is a good way to reduce your chances of actually getting it. Guild leaders have enough on their plate already without having members buzzing in their ears about why they should be promoted. The more you bring it up, the more likely the leader will express doubts about your sincerity and readiness for the responsibility.
Fill a need
Be proactive in looking to help fill the needs of the guild. Perhaps the guild is running low on certain resources or needs someone to offer class tips and advice. Maybe the guild needs someone with good clerical skills to help manage the guild roster, loot system, or event calendar. Or maybe the guild could use your coding skills to improve the guild website and forums. There are many tasks, both in game and out of game, that you could step up to help manage.
Also, consider some of the qualities that make a good leader and use your leadership strengths to assist the guild. You might have a good personality to help diffuse personality clashes. Or if you have a good relationship with the leader, you might be able to help out by being one half of a good cop/bad cop duo that is effective in enforcing rules with less drama. There's no one type of officer, and in fact, guild leaders benefit from a variety of personalities and philosophies among the leadership team. Just make sure to run things by the guild leader first. You don't want to conflict with what's already being done, and it's possible that someone else is already working on your project behind the scenes.
Work within the system
When you do identify a task you want to handle, make sure it's one that fits the vision of the guild. An issue that comes up often (and can be potentially disastrous) is when a member wants to nudge a casual guild into one that's more hardcore and focused on endgame. He might have the best intentions, but pushing a guild to be something it's not can create new problems for the guild leader. It also creates a tense environment, with members caught in the middle of having to take sides between the leader and the member who's lobbying to up the pace of the guild. Recruiting is another potentially tricky situation. You might want to get out there and help increase the roster, but a sudden influx of new members can upset the atmosphere of the guild and create personality clashes, especially if they're not screened properly. The best approach is to work closely with the leader and other officers so that you're keeping them in the loop on what you're doing and also making sure you're staying within the system and guild style that the leader has worked so hard to establish.
When you roll up your sleeves and get to filling gaps, do it quietly. Don't advertise to everyone how helpful you are or constantly announce everything you're doing. Believe it or not, people will see your actions without your needing to mention it, and the more low key you are about your contributions, the more that others will recognize you as a good leader who deserves that promotion to officer.
Don't rush it
Even if you've been a member for years and feel you've been loyal enough to get the promotion, you can't rush things. The leader has to have trust in you, and seniority shouldn't be the main factor in who gets to be an officer. Complaining or nagging will only erase all the positive contributions you have made to the guild. And be careful what you wish for! If you're a really good officer, you'll have plenty of tough decisions and difficult situations to handle, and few, if any, would want to deal with that.
Ultimately, the best way to earn the rank of officer is to put your head down, go about your business, and go the extra mile to support and improve the guild. Even if you really really want to be an officer, you're more likely to get it if you don't show it. And whether you get promoted or not, your actions will help improve the guild in a positive way so that everyone benefits. In fact, longtime guilds almost transcend ranks because everyone's chipping in regardless of rank. Some even rotate officers on a regular basis, which helps dispel the notion of officer as a power position. The bottom line is that it's not so much about the rank as it is about everyone contributing to make the guild both fun and successful at the same time.
Do you have a guild problem that you just can't seem to resolve? Have a guild issue that you'd like to discuss? Every week, Karen Bryan takes on reader questions about guild management right here in The Guild Counsel column. She'll offer advice, give practical tips, and even provide a shoulder to lean on for those who are taking up the challenging task of running a guild.