Being sweet to your girlfriend will almost always be appreciated, and can help reassure her that you're enthusiastic and positive about your relationship. Often it's the little things that can be the sweetest, as these help emphasize larger feelings of trust and intimacy between the two of you. Fortunately, it's not particularly difficult to be sweet to your girlfriend, although you should always be attentive about future opportunities to display your affection.
Pay attention to her needs. In different situations, this can allow you to display considerable acts of kindness, for example, lending her your coat when it's cold outside, or offering to give her a "piggy back" if she feels tired during a walk.
Give her spontaneous gifts. Although you shouldn't regularly spend all of your money on your girlfriend, the occasional little gift can help show her that she's been on your mind. This works especially well if the gift is relevant to a small detail she told you; for instance, if she mentioned a favorite book from her childhood, it might make a sweet gift to buy it for her if you happen to stumble across an old copy.
Clean up. Little acts of cleanliness can go a long way toward showing your girlfriend that you care, and can help demonstrate that you're attentive to making things nice and presentable for her. However, avoid doing this inappropriately; some people may have a certain style of organization that can be disturbed by well-meaning efforts to tidy up.
Bring up problems she previously mentioned. This can help indicate that you were paying attention, and that her troubles are important to you. For instance, if she mentioned that she was having difficulties with a co-worker, ask how things have progressed a few days later.