Break a large project into smaller assignments with intermediate checkpoints. For example, require students to submit a research proposal with a working bibliography, followed later by an annotated bibliography and/or research log.
Try the assignment yourself. Does the library have the materials you are suggesting to your students?
Be clear about resources you want (or don't want) your students to use. For example, if you limit the number of web sites a student may use, make sure it is clear to students that online journal articles are not considered "web sites."
Make assumptions about the skills your students bring. Discuss possible means of assessing their level of expertise with the subject librarian so the session can be tailored to what they need.
Make assignments that require all students to use the same resources. This leads to problematic access and potential mutilation of library materials. If you need all students to use something, place it on Reserve.
Give "scavenger hunt" assignments that have students hunting for random facts or pieces of information. These tend to be frustrating for students and library staff.