Maintaining a state of ketosis is as simple as following the same dietary parameters that got the body into that state in the first place. If you limit carbohydrates and sugars in your diet, your body will opt to burn the healthy fats you consume for energy. However, when cutting carbs from your diet it is very important to focus on limiting net carbs, which is simply the number of grams of total carbohydrates in a portion of food minus the grams of fiber. Constipation is a very common issue for new adherents to the ketogenic diet, and it is caused by foregoing fiber in an attempt to limit total carbohydrates. So to prevent constipation from derailing your ketogenic diet, simply make sure you consume adequate levels of fiber!
Weight loss is the primary reason my patients use the ketogenic diet. Previous research shows good evidence of a faster weight loss when patients go on a ketogenic or very low carbohydrate diet compared to participants on a more traditional low-fat diet, or even a Mediterranean diet. However, that difference in weight loss seems to disappear over time.
Most low-carb diet authors don't recommend bothering with it. Even many of those who think a ketogenic diet is a good thing just assume that a very-low-carbohydrate diet (under about 50 net grams of carbohydrate) is ketogenic. On the other hand, many people have found that monitoring their ketones, at least for a while, provides valuable information.