A good referee has appropriate experience and a high regard for your past performance and future potential. Ask if they will be supportive, not just if they are willing to complete a form on your behalf. Avoid using a referee who advises you to seek somebody else. If your referees are required to complete confidential referee reports, ensure that they return the form themselves. Attaching them to your application may make them void.
Timing is important.
Academic staff members in today's universities are very busy people and are commonly driven by deadlines. If you give them forms to complete too early they may be misplaced and forgotten. If you provide them just before the deadline they may feel less inclined to give you a favourable report. If you ask a referee to complete more than one form, supply all of these at the same time. It is usually worth following up your referee a few days before material is due.