I'm going to address this to guys because in general they need to hear it more than girls, but it applies to everyone:

Don't tell your belle that you love her until you're engaged. That thing about being "honest" and "open" and "just telling her how you feel"? Almost guaranteed she'll be off like a shot unless you're already deep into a relationship.

A few key points:
*ATTRACTION IS NOT A CHOICE.
*Humans are intrinsically drawn to what they can't have
*Dating should be about *having fun*
*Clingy guys are not fun. Telling her you love her too early is CLINGY. GIRLS HATE CLINGY. Guys do too, but usually they don't have that problem unless they're mac daddies (like me ~__^). Also, expressing one's affection for the other outright too early kills the chase. When the thrill of the chase is completely gone...the attraction gone.
*FLIRT! Flirting is fun for both parties. Relationships that aren't fun...fall apart. My grandparents are in their eigties, they've been happily married for over 50 years, and they STILL flirt. The thrill of the chase I mentioned? Flirting is humans' natural way of keeping that chase going even after years of commitment.
*This is going to sound sexist, but sorry ladies, it's true: girls are, with *very* few exceptions, attracted to a guy whom they perceive as higher status than themselves. When you treat her like a goddess...game over. Sure, she'll often accept the gifts and expensive dinners, etc. Who wouldn't? But you can give up on ever making her feel that gut attraction for you, bro. Ain't gonna happen. Take a clue from Han Solo. Seriously.

I could go on for hours, but those are the basics of Why Not To Say I Love You Until You're Practically Married.