Some Tips for Gentlemen
Personal Grooming is Important
Good hygiene is a must. Make sure to bathe and groom yourself regularly. When applying deodorant or cologne, don't use a lot. Some girls may not like the smell and if you reek of it they're not going to be pleased. Also, make sure you don't use too much hair gel (it's sticky and goes rock hard after a while).
Pay attention to how you look. Wear clothes that fit your body well, eliminate flashy clothing and avoid fashion "faux pas" (e.g. baseball hats cocked slightly to the side). A simpler wardrobe will keep the emphasis on you, not your clothes, which will change your outlook. Get regular haircuts.
A true gentleman is polite to everyone, in action and words. 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 (e.g. 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 stupid, be polite and respectful. Be prepared to ask questions or make statements of courtesy, like "how was your day", "may I help you?", or "let me get that for you", or "I'll take care of that". Don't swear. Ever. Don't speak of vulgar matter. If it's too difficult to stop cursing all-together, tone it down a lot.
In public, don't: pass gas, belch, speak too loudly, be overly demanding/offensive, or drink heavily. A gentleman is always in control of himself, body and mind. When walking her across the street, hold her hand. It gives the woman a sense of feeling protected.
Partake in Polite Conversation
Don't talk about yourself much. On the other hand, girls like to know a little bit about you so they can understand what you are talking about sometimes. In addition to making you a much more desirable conversationalist, it will make you seem more mysterious to not share too much, which many women find attractive.
Get a basic idea of what's going on in music, sports, and politics. Learn to avoid politics 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.
Be Courteous to Women
Don't stare at and ogle women or say anything degrading to or about them. It's never a good idea to objectify a woman (or anyone else, for that matter). Give girls a little extra respect. Don't be obsequious, but neo-feminism has kind of gotten rid of some basic politeness that they'll appreciate, such as opening car/building doors, etc. 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.
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 into sports or video games, you can say a few things but then move on to something she's into. 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.
When walking on the sidewalk or on the side of the road, gentlemen should walk on the outside(closer to the road), whereas the lady should be on the inside, protected from oncoming traffic. Do not talk down to women ever. It's okay to tease but being cruel is not the way to go. Teasing does not mean calling them vulgar words, however. No matter how joking your tone is, it hurts a woman when a so-called "gentleman" calls them a dirty word.
Pay attention to the fact that the world is more threatening for girls: they're automatically targets at night or in bad neighborhoods. Walk her to her door (or car) 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. If your girl has to park far away from her dorm or home, always offer to drive her to and from your destination. She will appreciate how caring you are about her safety.
Respect Your Girlfriend
Remember to keep doing nice things for your girlfriend when you're around her. If she's carrying something, pick it up when she puts it down and always kindly let her know by saying "oh, let me get that for you", whatever the object is. Remember, being selfish is not attractive.
If you are watching television with your girl and you know she likes a particular show or sporting event, leave it there. She will appreciate that little bit of selflessness more than you know. Show up with a card or a flower and not just for holidays. Expensive and showy doesn't matter, it's the effort. In fact, a rose, a little love note left on her pillow, or a heartfelt kiss will keep any girl happy for days.
If in love, be sure to tell her often that you love her and make it sound meaningful. Be open about yourself to her and share with one another, making sure you laugh and keep things upbeat. Don't be a stiff bore. Stand up for your girl. Don't go around punching everyone who looks at her sideways, but if anyone is 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.
No Fighting Allowed
Avoid fighting, as it isn't the best way to solve problems, however a real gentleman never backs off when he's challenged, always standing up for himself and his values. Remember to prioritize and not sacrifice your values for someone who is clearly just picking a fight with whomever will join in. If someone is simply embarrassing himself by offending you, don't sink to his level, but walk away.
If you have to fight, keep the straight posture, even if your rival is taller than you. Be reasonable, never use weapons, such as guns, knives and the like, as this kind of "chivalry" is from the old days and might end up seriously damaging you. (Unless deadly force is being used against you or you can articulate it is imminently going to be used) And never, never fight in/against a group, for a gentleman only duels, do not try to be Chuck Norris. If your opponent is down, that means you made your point and it's time to stop fighting. Offer him a hand to get up, always expecting a bad reaction, as he might not have the same values as you have.