If you didn't understand my question let me explain it to you: I have a sidebar with my newsletter form that says something like: "subscribe to the newsletter to get...". As it is in the sidebar this text and also other text appears on all pages of my website and forum therefore being duplicate content. How can I avoid duplicate content this way? I don't want to remove it obviously . I read somewhere that adding more text than I have in my template is a solution but in my about-us and disclaimer page I only have about 2 sencences. What should I do?
Those sentences in your sidebar are insignificant and would not be flagged as duplicate content when compared to the number of words on the entire page. Increasing the word count on the page itself is one approach but usually sentences like "Subscribe to our free newsletter" and so on are so widely used on the net that Google will not penalize you for it.
The search engines are generally smart enough to figure out the content blocks of your website and won't consider them as main content of the site. While it is duplicate content, you won't likely receive any sort of penalty or filter because of it.