The trick here is not only to avoid all obvious sources of carbohydrate (sweets, bread, spaghetti, rice, potatoes), but also to be careful with your protein intake. If you eat large amounts of meat, eggs and the like, the excess protein will be converted into glucose in your body. Large amounts of protein can also raise your insulin levels somewhat. This compromises optimal ketosis.
Sure, you can lose weight quickly. There are plenty of fad diets that work to shed pounds rapidly -- while leaving you feeling hungry and deprived. But what good is losing weight only to regain it? To keep pounds off permanently, it's best to lose weight slowly. And many experts say you can do that without going on a "diet." Instead, the key is making simple tweaks to your lifestyle.