hm, it always depends on what it is supposed to protect you against.
There are a couple of pitfalls you have to be aware of:
-posting of copyrighted material: You as the owner of the forum might be held liable if you or others post copyrighted material like newsstories taken from news websites, etc. A simple disclaimer won't shield you from liability, you have to make sure this will not be tolerated. While you cannot know in advance what other people may post, you must delete it if it is in violation of copyright.
-defamatory postings: Again a simple disclaimer won't necessarily shield you from liability. While there is a constitutional right to free speech this right is limited by the right of others not to be defamed.
-pornography and other offensive postings which might be against the law
-other unlawful kinds of postings: invasion pf privacy, disparagement, etc.
You should draw up clear rules against these unlawful things, post them conspicously on your forum and make sure that anyone who registers accepts them. And then you must diligently look for unlawful postings and act accordingly.