Design a matrix that measures key elements under assessment. Be sure that the matrix incorporates features that require the assessor to read the questions carefully and completely. For example, do not put all of the positive evaluations on one side of the matrix. A good matrix should have some questions that "strongly agree" and others that require "strongly disagree" to produce a high score.
Include open-ended questions that require textual responses. An example might be "what is the most positive aspect of this colleague's work product" rather than "how do you rate this coworker's productivity." The former requires a qualitative response, while the latter question might be completed with a one-word answer.
Ensure that peers are evaluated by coworkers who are relatively neutral in their opinions. Ideally, peers should request evaluations from those coworkers that they feel will best represent their work product, but this may result in highly biased evaluations. One way to deal with this is to appoint multiple reviewers or assign them randomly. If your organization is large enough, you might solicit peer reviews from individuals outside the department who have general familiarity with the nature of the work involved.
Allow employees access to the written reviews and give them the opportunity to submit written responses to be included in their evaluation files.