Be true to your girlfriend. The base of any relationship is dependant on how honest you are to each other. It also builds up the respect for each other. However, be tactful when situation demands. At times, the reality is not well accepted by the girl when it comes to dressing sense, looks, style, etc. On such occasions tell them in a way that does not sound derogatory to them and conveys your message as well.
Most of the girls suffer from inferiority complex. Do not let this feeling overpower your girlfriend. Let her play an important role in all the decisions you take in life which is meant for both of you. If not, be prepared for the emotional game as girls often resort to this when they are ignored or not heard.
Communication is very necessary for any relationship to survive. Talk to her and let her know about what you feel, what are your aspirations, what she felt about a particular thing which happened, etc. give them the vibes that she can confide in you everything and would not have to worry about it.
Girls love to be pampered and when it comes to boyfriends they want it all the time. Pamper her with gifts, soft toys, dinner, flowers, cards, CD, books, etc. You do not have to get expensive items each time. Girls get impressed with even the simplest of all things like a single rose or a small key ring. So make efforts to make her feel special and loved.
Romance holds the key for your relationship. If you are romantic, sweet and thoughtful, your relationship would never suffer. Gesture speaks volumes about what you feel for the person. Compliment her, leave her a romantic note, call her unexpectedly just to say those three golden words. Even if both of you have been together for a long time, you need to make efforts to keep the romance alive.
Be interested in her interests. Girls love to talk and want someone to listen to them with a patient ear. It is noted that mostly guys do not take interest which usually leads to fights and tiffs. Show interest in what they say. Do not just blankly listen but be an active participant in the talks. Make them feel that something which is important to them is also important to you.
Do not be clingy and give her space. Being in a relationship does not mean that she has no personal desires. Let her be a free bird and move about freely.