Cheering-up Kiss

If your girlfriend is sad (works best if she is crying) for any reason, the first thing to do is to put your arms around her. Let her hold on to you, and whisper sweetly in her ear.

Either that, or you can pull her into your lap and hold her around the waist from behind. Nuzzle the side of her face. Whisper in her ear. Be sweet.

Lean over her protectivly. If you're sitting, lean in and hold her tightly. Let her know you only want to help, let her know she can trust you.

When she has calmed down, gently kiss her head, cheek, or temples. DO NOT kiss her on the lips or neck. It may make her feel uncomfortable. DO NOT kiss her for too long. Don't make it a peck, either, but just right. Be gentle, loving and sweet the whole time.
The first time I did this wasn't with my girlfriend, but a very good friend/crush when she was really freaked out by soemthing.