I've been "maintaining" (I guess) for probably 18 months or so. I call it "getting better", but whatever. Actually, to me, maintenance is quite simple and that is ... PAY ATTENTION!
Most people hit a goal weight and go back to doing what they did before. It's the same as wearing glasses for months to correct your vision and saying "Well, I've been seeing great now for 6 months, I don't need these flippin' glasses anymore" and chucking them out of your moving car.
So back to paying attention. Set a weight range that is acceptable. Say 10 pounds in either location of your goal weight. If you're getting close to violating that 10 pound threshold, change something. Start tracking, step up your exercise, etc. If you're within that 10 pound threshold, continue on with a little less strictness.
Continue logging into sparkpeople!
Finally, never stop setting fitness related goals. This is particularly important in maintenance. Since goals are typically (and foolishly, IMO) set around a number on the scale or a clothing size during weight loss, you don't get to do that during maintenance, so most people get lost. Without a goal, they lose interest and go back to old habits (which equals weight gain for more often than not).