Consider what type of roommate with which you would be willing to live. Make a list of requirements such as gender, smoking or non-smoking, student or non-student.
List other important aspects such as cost of rent, pets or no pets, and distribution of household chores.
Write an ad. Be sure to include where you live (this can be general area), contact information, available hours, and the information you have determined to be important from your generated list of roommate qualifications.
Photocopy the ad and tape it on bulletin boards available for announcements all across campus, especially those in high-traffic areas.
Post the ad on college forums and blogs.
Wait for a response. Don't forget to keep listing on available forums to keep up exposure.