Much depends on the character, temperament, habits of girls. Feel free to talk about their feelings. Even if a loss or embarrassment, the words come out not the most eloquent - nothing! Importantly, they were sincere. Remember that 99% of girls are willing to endlessly listen to the assurance that they are the best, beautiful, good, desirable. This is the case when the "porridge oil will not spoil".
Do not forget the proverb: "words – words and deeds – deeds". Even small signs of attention given to the girl in front of her to open the door, shake hands, leaving the bus or tram, to present a bouquet of flowers or a box of chocolates, to cook for her a tasty dish), operate properly.
If you convince the girl that she for you the best and favorite then all your previous novels have to be "a mystery". In any case, not to speak about the girls I loved before! Well, if such a conversation started (on her initiative), be sure to reassure, and as convincingly as possible that it is much better!
Feel free to make plans for a joint future life, albeit in the form of assumptions. Since even the most independent she realizes that she needs to get married and have children. Therefore, any fan she instinctively considers in this respect: will he be a good husband and father? Your words will help to convince him of the seriousness.
Although it is not the old days, but the desire of the guy to meet the parents of girls also serves as a good proof of the sincerity of his feelings. So be sure at the first opportunity, ask the girl to introduce you to them. Well, try, of course, to make a good impression on them.