"I love you"
Yeah, this is very good for you if you are in a LTR. When a girl says "I love you" to you without you saying it first (as it should always be done by the way), then she is letting you know that she wants a LTR with you.
The age old principle: Girls want what they can't have.
So, you need to not say it just yet. My best policy is to say I love you to her with your eyes and kiss her passionately when she says it to you. But don't say IT! Make her preen the phrase out of you with nagging, coaxing, manipulation, begging, and other famous female behaviors.
Trust me on this. Us, as guys, are vulnerable to that phrase. Girls fall in and out of love as fast as the seasons change but us, no. When we start telling a girl that we love her, we begin to concede to her feminine powers. This is the truth. It is one of the qualities that make us male. When we say something, we mean it. As a result, we are also much much more hurt by it if the relationship ends up not working out in the long run. Saying "I love you" means more to a guy than it does to a girl. We can't just get a haircut and forget about a relationship.
When you make a girl have to work very hard to preen the "I love you" phrase out of you, you are increasing your value to her by being a challenge. You are telling her "You are going to have to work a little harder to win me over". She will keep telling you that she loves you. She will keep bugging you by hinting at the fact that you haven't said it to her yet. But deep down inside, she loves you for it because she knows that you are valuable and will not give your heart away to her feminine powers so easily.
Of course, you can take this too far. The key is to not SAY it but to show it. Get her small gifts here and there to let her know you are thinking about her (provided that you are).
Don't say "I love you" until you really actually totally mean it. When she says it, SHOW her that you love her back. Be creative. By saying it, you are giving up some of your freedom so make her work to hear it.