While it's no secret that sex is important to the male species, sometimes it's a bit tough to identify just what men look for in a girlfriend -- we're talking long-term relationship material here.

Tapping into the knowledge of two relationship experts, we've come up with a list of 10 characteristics that will take a man from "just a date" to "meet my girlfriend".

1. A sense of humor

"Nobody wants to date 'Debbie Downer' and men enjoy joking with their women," says Zachary Davis, author of Roses and Brass Knuckles: A Man's Pocket Guide To Winning The Dating and Marriage Game. Clearly, this is something that men and women can agree on. You don't want to spend your time with someone who is seriously serious so be sure to lighten the mood when you can. Men like it when a women has the confidence to be funny. "Believe me, we want you to make us laugh too," says Davis.

2. Flexibility

No, we're not talking about physical flexibility (though, no doubt, plenty of men have that on their list). We're talking about the ability to be adventurous and explore life as it happens. Julie Spira is a best-selling author, radio host, columnist and dating/relationship expert. She believes that "men love it when a woman doesn't mind if the plans get changed last minute. Spontaneity excites a man." Last minute plans from a movie night to a pizza date? Don't panic. Go with the flow and impress him with your carefree spirit.

3. Honesty

No long-term relationship can survive without honesty and men want to trust their girlfriends. "Women don't like to admit it, but it is a lot easier for you to deceive us than it is for us to deceive you," says Davis. "We don't pick up on a lot of the signs that women do, so we have to be able to trust you fully." If you really want a solid relationship, leave your affinity for little white lies behind you.

4. Positivity

There's no question that life has its ups and downs but an emotional roller coaster is not high on any man's list of desired traits in a girlfriend. Regardless of what life throws your way, a positive outlook on life will get you far. "Too much doom and gloom usually works its way into the bedroom," according to Davis. "How can we have good night if you are always having bad days?"

5. Support

This can apply to both his personal and professional life, but men want a woman who is in his corner no matter what. "A man's self esteem and ego are driven by his success in the business world," says Spira. "When a woman admires a man for his work achievements and becomes his cheerleader, she's on the top of his list." Take an interest in his work, make an effort to understand his struggles and then provide encouragement.

6. Reliability

It's doubtful that an unreliable woman (or man) will change her (or his) stripes once a relationship is in full swing, so men want to see this trait in a woman before things get serious. "Just like women want a man they can count on, we want the same in return," says Davis. "It is easy to love when you can count on someone!" Some ways to show that you're reliable include being on time, following through on commitments and being a good friend.

7. Motivation

Motivation is usually high on the list of characteristics that women look for in a man, but it goes both ways. "How can we move forward in a relationship, when you don't want to move forward in your life?" asks Davis. Openly sharing your professional and personal aspirations will show him that you have big plans in life (that may or may not include him).

8. A social life

Even men who enjoy being in a relationship need their space, and they need to know that their girlfriend can have fun without him if necessary. "Nothing is worse than when you starting dating a woman and she becomes completely attached, and the two of you are together 24/7," says Davis. "Then we start to think, what was she doing before we met? Time apart makes the heart grow fonder, even in the beginning." Don't even think about turning your back on your girlfriends just because Mr. Right comes along. Chances are, in the short term, he won't be pleased and in the long-term, you won't be happy.

9. Sexy + Smart

We all know the first characteristic tops a man's list, but combine sexy with smart and you have "girlfriend material" written all over. "The combination will send a woman to the top of the list," says Spira. "Men are visual. When they see a woman with the intelligence behind the pretty face, it's a winning combination." Don't hesitate to share your intellectual passions. Maybe you are a crossword puzzle buff, an economic genius, a business whiz or aspiring author. Whatever your area of interest, flaunt it; don't hide it.

10. Confidence

Again, this is something men and women have in common when looking for a long-term partner. "A man is attracted to a woman who is confident on their own," says Spira. "Although they look forward to sharing together as a couple, if she can make it on her own and isn't needy, she'll capture his heart." Davis agrees saying, "If you don't love yourself then I can't love you. Simple as that." Take pride in your abilities, your body and your life and men will find you hard to resist.