Encourage A Conversation
She needs to get whatever is upsetting her off of her chest, so encourage her to have a conversation about it -- and don't just keep asking her what's wrong.
Make Her Laugh
Having a sense of humor gets you pretty far with women, but making her laugh won't always work. In fact, sometimes it's downright dangerous -- if she's seriously upset, then trying to make a joke out of it will only double your trouble.
Buy Her Flowers
For maximum effectiveness, send flowers after you've already talked things over with your partner. Since you're already partly out of the doghouse, the gift will be more unexpected, thus winning you bonus points.
Cook For Her
Cooking for her is one of the most foolproof ways to get out of the doghouse. Surprise her with your culinary skills. Even if she's raging mad at you, it will be hard for her to stay angry if you've taken the time to make her favorite meal. You'll score extra points if you aren't typically the cooking type.
Listen To Her
Listening is a skill. Let her talk, and pay attention. It's important that you resist the urge to interject. Don't interrupt or try and tell your side of the story. Just shut up and listen.
Conversely, sometimes the best thing you can do is walk away. Leave the room. Leave the house. Give her some space and time. We're not suggesting you storm out while she's in mid-sentence. Just recognize that if she's furious, there may be no reasoning with her.
In the end, nothing beats a genuine "I'm sorry." A sincere apology is key to getting out of the doghouse, and you're probably not fixing anything with her without one.