The first and most important criterion for any reference is the level to which s/he knows you. The more specific the writer can be in relating skills and potential (i.e. including examples and anecdotes to support the board, glowing statements, the more effective s/he can be in writing a strong letter or recommendation.
Secondly, the writer must have a favorable judgment of you and your abilities. If you are uncertain about this, ask the writer if s/he can write a strong letter of endorsement for you, not just "a letter or reference."
Thirdly, it is important for the writer to know a good deal about the type of employment or course or graduate study you are seeking. This allows the writer to speak with authority when professing that you are well suited for a particular line of work or study. HINT: Provide your references with a current resume to assist them in their task of writing a letter.
Generally, you should have three to five letters of recommendation on file. You may solicit more letters and then choose particular letters for different positions or schools. When an exact number is requested by a school district or hiring agency, send only that number; sending more or fewer indicates failure to follow instructions. Ideally, references should be obtained from both your academic professors and previous employers or supervisors.
Sometimes, names of several people who may be contacted directly to discuss your back ground and qualifications may be requested. It is good practice to provide your interviewer with a list of names, title, addresses, and telephone numbers of at least three people who have agreed to provide strong, verbal references for you. These may be the same people who have provided you with written letters of recommendation.
GOOD LUCK! Remember, luck tends to favor the well-prepared.