1. Pay attention to how you look. Wear clothes that fit your body well, and eliminate flashy or obnoxious clothing (i.e neon orange basketball shoes) that make you look tacky. A simpler wardrobe will keep the emphasis on you, not your clothes, which will change your outlook. Get regular haircuts.
2. A true gentleman is polite to everyone. Always be aware of how you can help people. Wait an extra few seconds to hold the door for the person behind you. Offer to help an old or pregnant person get their grocery bags to the car. You don't have to be paranoid or risk killing yourself (i.e if you've got a precarious grip on something heavy, don't open the door for someone) but respect is paramount. Even if someone is being a jackass, be polite and respectful.
3. Don't swear. Ever. Growl wordlessly or calm yourself down.
4. Give girls a little extra respect. Don't be obsequious, but neofeminism has kind of gotten rid of some basic politeness that they'll appreciate. Don't put every girl on a pedestal (we're all human) but they'll really appreciate it if you do them favors, like giving up your seat on the bus even though they're not carrying anything heavy.
5. Don't talk about yourself much. In addition to making you a much more desirable conversationalist, it will make you seem more mysterious which will help you get girls. Get a basic idea of what's going on in music, sports, and politics.
6. Learn to either avoid politics and religion until you know someone better, and learn how to be neutral if someone else brings it up. A simple shrug will do wonders. A gentleman doesn't make other people uncomfortable.
7. Something guys don't really appreciate is that the world is more threatening for girls: they're automatically targets at night or in bad neighborhoods. Walk her to her door if at all possible. It doesn't matter how strong she actually is, if you're a girl people will go at you first, whereas by virtue of being male you're safer.
8. When you're with your girlfriend, remember to keep doing nice things for her. If she's carrying something, pick it up when she puts it down. Show up with a card or a flower. Expensive and showy doesn't matter, it's the effort. In fact, a rose and a heartfelt kiss will keep any girl happy for days.
9. There are some things you shouldn't talk to girls about. These include how hot other girls are, or anything at all of that nature. If she's not in to sports or video games, you can say a few things but then move on to something she's in to. Girls tend to be more interested in books and music than in sports and video games, though there are plenty of exceptions to every rule. It's fine to talk about that stuff with your guy friends, but a gentleman doesn't do that.
10. No matter how horny you are, let the girl set the pace. You should love her for who she is rather than how she looks.
11. Stand up for your girl. Don't go around punching everyone who looks at her sideways, but if anyone's giving her looks or unwanted advances, intervene. Put your arm around her shoulder and move yourselves away, or go to her and speak up for her. The physical contact reassures her and lets the aggressor know that he's got to deal with both of you.
12. Always have a straight posture (nobody likes a sloucher)
Tips

* Be sincere.
* Basic hygiene is important but a gentleman should go a little farther. Carry a pocket comb and keep your hair neat. If you have a more tousled style, keep it out of your eyes.
* Looking good doesn't mean you can't keep wearing jeans, T-shirts, and sneakers, it just means that they should all be clean, unwrinkled, look good on you, and not be offensive. However, at the end of the day, it's the guy inside the clothes that counts. It's also good to wear a belt, it can do wonders
* It's not as hard as it sounds.

Warnings

* Don't use illegal drugs or drink alcohol to excess. They will impair your judgment, and a true gentleman is in control of himself at all times.
* You're not better than anyone else just because you're doing this. Don't think it, don't act like it.
* While being helpful is nice, don't be overbearing. Some women might not like someone hovering over their shoulder every minute.