If students don't see the value of peer review, they are unlikely to spend much time reviewing others' drafts or to take peer advice seriously. The most effective way to encourage students to take peer review seriously, both as the reviewer and as the writer, is to include effective peer review as part of the overall grade for the assignment. Skimming peer review comments will take just a few minutes (even for multiple reviews of complex documents), and you'll quickly see which students provided the most helpful commentary. Alternatively, you can ask students to rank their peer reviewers and base the peer review part of grade on peer ratings.
If you're uncomfortable weighing the quality of peer reviewing in the assignment grade, consider dividing the course grade to include a separate class participation or peer-reviewing grade.