The first thing I really want to highlight, is what Army Fatigue looks like. It can take many forms and in differing severity. The main one that you'll see is just plain boredom. You might be tired of painting space marines or just don't like how they play anymore. Maybe they are just too basic or good for you. So you stop playing them. What then? Another is something I call Bystander Syndrome. This is where a player who has an army watches games, but won't play them. Often this player will put out advice for the players in a constructive way, but often it is unwelcome and unproductive. No matter how much you ask them to play, they just can't bring themselves to bring the army out of the box to game with.
The gradual slip of interest in one's army can be catastrophic to the gaming community if they don't remain in the game. Every player counts in this game. Sometimes a player can start to lose interest in their army and/or 40k because they are playing poorly and constantly losing. Nobody wants to be the loser, so if they don't win at least some of the time, they will often leave the game. I've seen it before several times. The same can be said of a player who always wins. Yeah, its fun to win, but unless your opponents are good and give you a run for your money, boredom quickly sets in.
So what are some ways people get around this troubling problem? I personally took a more expensive route when I felt fatigue set in with my Black Templars (though I will never give them up completely!). That was to start a second army, which happened to be the Dark Eldar. Often times, a person will start new armies as codecies are re-released by GW. Recently, there have been a huge influx of Imperial Guard players that never existed before, because of the strength of the codex. When asked what their previous army was (if any), it was often a very out-dated xenos codex. A second army allows you to play a different style of game than your first army, with different paint schemes, models, and tactics. But its not for everyone, especially due to time commitment of learning the new army, painting and modelling them up, and the costs of buying the army itself. Sometimes I think GW has army fatigue planned into their business model, as this is something they hope everyone experiences.
Another way to beat the fatigue back into a cave, is to become a mentor to a new player of your army. It is always a good thing to pass on your experience to a new player to help them adjust to the game and their new army. The problem with this, is that new players do not know if that advice is good or not. Obviously, the veteran does or they'd have not said it. But if it contains bad advice, you risk starting the new player down a path of poor game play after they spent hard earned money and time building the army up. This can break new players early, and unfortunately, it happens more often than I care to think about. But if you know your stuff, and have the support of others in the community, go for it! It was one of the reasons I built this site. You can also engage in the online 40k community to keep you interested. There are a great many sites, forums, and blogs that are constantly putting out great information and discussions in which to keep a player engaged.
You can also ease boredom by developing fluff for your army. Right out of the codex, an army is full of fluffy goodness already. But that doesn't make your army very unique, as you could be drawing your forces from any number of those no-named members. Developing personalized background for your army often will help you repair the roots which drew you to the army in the first place. Except you get to decide why the army exists as it currently does. Themed armies are also excellent, but sometimes they are not very competitive. But if you don't mind that, themed armies are great and full of character! For me, just personalizing my army with a background that is unique to only my crusade and painting them to match makes me even more proud to field them on the table top. :)
One of the best ways to stay interested in your army is to play with it in a competitive tournament if you have the opportunity to do so. I will note that you have to have a certain set of skills when doing this such as list building, tactical thinking, sportsmanship, and problem solving. While there is a great debate going on over what tournaments are actually competitive or not, but in general, participating in a tournament is a great method of testing your skills as a general of your army/race. It is the end goal for competitive players in my opinion. While it might be good for some, I certainly don't want to put huge amounts of time into building, painting, learning my army just for it to sit on a shelf. I want to play people I've never met before and develop that connection that comes from competitive game-play.
Hopefully my little article was insightful and can assist a player decide