When You Have Nothing Positive to Say
It's actually in the person's best interest for you to politely decline writing a reference letter if you can't provide more than a wishy-washy endorsement. A less than positive reference can cause as much harm as a negative reference. Employers are usually good at reading between the lines and will pick up on what you're not saying.
If you decline, the person can move on to another reference who may be able to provide a glowing recommendation. A simple way out is to say that you are not familiar enough with their work or background to provide a reference. That way you can minimize any potential hurt feelings. Here's how to decline a request for a reference.
If you're thrilled to be asked, but, not sure what to say, ask the person for a copy of their resume and a list of accomplishments. This will give you guidelines to use when composing a letter. Start by describing how long you've known the person and in what capacity. Include dates of employment and details on how you've worked with (or known) the person.
Continue by describing the person's skills and performance and what makes them an ideal candidate for a potential new employer. Also include two or three outstanding attributes. End by summarizing why you are recommending this person for employment. You may also want to provide a phone number or email address so employers can follow up if they have questions or want more information.