Prepare a standard list of questions you will ask each candidate. You can ask additional questions as they occur to you, but having standard questions will help with showing a lack of bias in your hiring decision–you can compare apples to apples.
Include both technical and behavioral questions, all open-ended. I’ll include my interview questions for entry-level computer operator as an example:
*What have you done in the last 1 year that would prepare you for this postion?
What motivates you?
When you find yourself bored, what do you do?
Describe a time when you had to respond to a deadline that you knew you couldn’t meet? How did it effect you?
What leadership style best suites your approach to work?
How would you rate your organizational skills?
From a troubleshooting standpoint, what skills and/or experiences do you believe you posses that will carry-over to this position?
What sorts of things upset or annoy you?
How much PC experience do you have? What operating system/word processing, etc.?
Do you have mainframe experience? What operating systems? What type of experience (end user, operations, administration)?
What, if any, has been your experience with Unix? What flavors (aix, hp-ux, linux, etc.) Give an example of some of the commands you use most frequently and a description of what they do.
What kind of experience do you have with networking? What kinds of equipment have you worked with? What are some common problems and their resolutions?
Have you worked with NT 4.0/Windows 2000/2003 servers? What about Outlook/Exchange? Any Active Directory experience?
Do you have any operations experience? What about customer service? Are you more the type who likes to work alone, or with a team?
Are you flexible about the hours you work? Will working different shifts on the weekends be an issue? Would you be available at all during the week if the opportunity presented itself?*
The more you can get them to talk, the more you’ll learn. Take notes, and use constructive silence to allow them more time to elaborate. Be enthusiastic and confident.
Be prepared to answer their questions as well, to the best of your ability. And for God’s sake, make sure you set an expectation of when they’ll hear your decision, and actually let them know if they’re not the Chosen One.