Well, it is simple. If you reach your desired weight and the weight is still coming off, you can increase your calories a little until you stop losing weight. Additionally, you can increase your simple carbs a bit, that will usually stop weight loss. However, this is *rarely* a problem. You must consider that surgeons advise you can expect to lose 70% of your excess weight after bypass. Many (me included) have set our goals at our 'ideal' weight according to BMI which would be 100% of excess weight lost. Your weight loss will eventually slow down and stop (somewhere between 1-2 years post-op) all by itself. People who have reached their goal weight and wish to stay at that weight after their body adapts may find that they have to work a bit to maintain the loss. I wouldn't worry about trying to stop your losses until after 1-2 years. At that point your pouch will have stretched some and you will have grown more intestinal villi to compensate for your shorter intestines so you absorb more of what you eat. That is why you will sometimes hear doctors and long term bypassers say that year 3 & 4 are the tough ones and the time when weight can fly back on.
Hope that helps,
Nic