If you are in one of those situations where you constantly are in fear of losing another member or suffer from this plague a bright idea is to start to concentrate on your momentum. I watch bands walk into situations where there is going to be month long gaps where they do nothing but sit on their hands, making no effort to grow their fanbase. This is when people leave bands. If you want this to stop happening start staring at that calendar and make sure things are happening and when a time of slowing down is about to happen, make sure to remedy with something that keeps the ball rolling.
Have you put Spinal Tap to shame in the member change department? Usually, if this is the case it comes down to one or two little things either there is a member with a major attitude problem or you constantly lose momentum and people get bored.