Don't feel as though you need to be over the top in everything you do. A well-developed, nuanced character played consistently is much better than a wild and crazy character who's all over the place.
Always agree with your fellow improviser. Remember the rule "yes, and." For example, if your scene partner says you're in a swamp, don't contradict him by saying you're actually in a school. This will essentially end the scene and make both of you look bad. Instead, accept what he's said and add your own details, such as saying that the swamp is full of alligators. This raises the stakes and keeps the scene moving forward.
Make the scene about the present. The audience will be more engaged if the action on stage is urgent. Simply recounting what happened to your characters last week will not keep an audience's attention. However, it can be good to give your characters history by mentioning clues about their age, location, and motives. A character with a wife and kids who lives in Alaska will do things differently than an Australian kid who's failing out of school. Your character's history will help inform their actions and make them more believable.
Be consistent, and remember the things you have said and done within the scene. For example, if you establish that there is a window at stage left, that space must stay a window for the duration of the scene. Likewise, if your character mentions his sister, don't then talk about being an only child.
Listen to your fellow improvisers, and do not anticipate how they may react to certain things. This will help the natural flow of the scene.
Be brave and make choices. Don't worry yourself about "looking stupid." The rules of improv are completely different than everyday life, and as long as you commit to your choices you will not look bad. Similarly, don't be afraid to use your whole body. Improv relies heavily on physical humor, so don't let this resource go to waste.