Some people go into Netflix and simply add ratings onto everything they have seen, without realizing that the mood they are when they are rating shows and movies will often affect the rating they give; rather than simply tossing ratings around without any formula or system, it will be far more effective if you go in with a well thought out approach.
When you are rating shows and movies on Netflix, you should maximize your use of the three and four star ratings, saving the "extremes" (the one and two star ratings, and the five star ratings) for movies that you simply cannot get away from hating or cannot get away from loving, as this approach will help to ensure that you are only giving a movie the death kiss of "one or two stars" and are only praising a movie as "five stars" when you truly feel strongly about that movie. Netflix will be much better equipped to give you movies that fit your true tastes when you do this, as the suggestions they provide you with will be more greatly influenced by the movies you truly hate or love than by the movies about which you felt relatively neutral.
Once you have rated enough of the shows and movies that you have seen, you will begin to get suggestions from Netflix - and the more ratings you provide, the more accurate these suggestions will be - so make sure you take advantage of these suggestions, and allow Netflix to provide you with new movies that you can add to your favorites!