Focus on the Long Tail
During the first few phases of an SEO campaign the focus is usually on the major keywords and phrases. These will be linked to the most important pages on the website. It's usually the rankings of these key pages that plateau first.
When this happens, the best approach is to cast your net wider. This means focusing your efforts on the long tail.
Long-tail search rankings can often be just, if not more, valuable than your major keywords and phrases. They have lower competition, are more specific and, if chosen well, can be extremely action oriented.