Once you've reached your ideal weight, don't throw your scales away! They are useful for your stabilistion period, as they will send alarm bells off as soon as you let any weight creep back on.
Some people advise against weighing yourself very regularly, but by all means weigh yourself every day if you think it necessary.
If the weight does creep back on, you need to react quickly: the later you react, the more resistant your body will be to calorie reduction and exercise.
If you put more than 1.5 kilos (that's 3.3lb) back on, lauch a counter-attack by eating a protein-only diet for not one, but two days a week, until you return to your ideal weight.
If you put more than half of the weight you lost back on again, spend a few days of phase 1 of the diet, then re-do phase 2.