Always remember that this is an effort-dependent test. This means that the person being tested must be both able and willing to blow out hard and fast to the best of their ability.
This requires the test administrator (tech) carefully explain the nature of the test and what is expected. It is also helpful to assertively coach the testing subject during the performance of the test.
The best way to ensure that the subject has performed the test to the best of his or her ability is for the subject to perform the test more than once – at least enough times to obtain repeatability, if possible.
You can also look at the peak expiratory flowrate as a quasi measure of effort (i.e. if the PEFR is low but all other values are normal or near normal, the effort was probably poor).