IMS analyzes incoming messages on several key characteristics to prioritize email; including directness of the email, conversation participation and contact priority. To improve scoring accuracy, tweak the weighting that IMS gives to each of these messages characteristics via ClearContext > Options > Scoring Options.
For example, if you work in sales and receive a great deal of important email from new contacts who are not in your address book, you might want to adjust the Known Addresses weighting down and the Thread Participation weighting up to ensure that email conversations started by you and/or that you have participated in receive a higher score (and therefore higher prominence in your inbox).
For more detail on adjusting scoring and what message characteristics impact each factor, see our User Guide.