Two words: Sample Size...A power analysis is a must. By including a competent statistician on your team from the get-go, you will likely save yourself a great deal of frustration when you are writing the results and discussion sections of your manuscript or report.

It is all too common for a principal investigator to collect data prior to consulting with a statistician with the expectation of a "predictive model" or a "causal relationship" from a sample of less than 30 subjects. Had the PI consulted with a statistician prior to collecting data, the statistician would have been able to inform the PI, after appropriate analyses, to collect more data/subjects or to restructure the goals of their analysis plan/project.