If your like most guys, when you first start dating your girlfriend, you go out of your way to impress her. You buy her flowers, open doors, compliment her, etc. After a few months most of these actions fade away and you get in a routine with your girlfriend. To show your girl you really care for her you need to start doing a few little things for her now and then.
An easy way to show your girlfriend your thinking about her is bring a single rose next time you see her. A whole bunch of flowers is not needed. Giving her one simple flower for no reason at all other than to show you care will be a big hit.
Try to keep track of all the important events going on in her life. If your talking on the phone, and she mentions shes meeting up with an old friend, be sure to ask her how it went next time you talk to her. She might mention that she has a small headache. The next day ask how she is feeling. Remembering little things like these can go a long way.
Most of all remind her why you like/love her. If your with a girl for a long time, its easy to forget she needs to know you still really care about her. Tell her how you love her smile or the way she laughs. Go out of your way to make her happy.
Remembering to do these simple gestures as your relationship progresses; will keep your girlfriend very happy and make your relationship much better.