Secure letters of recommendation from people familiar with your work
The best recommenders will be distinguished former professors, contacts who can attest to your commitment, work ethic, and who also know the field youre applying to intimately. Often, your undergraduate and graduate professors and advisers will have a better handle on the specific information the admissions committee wants than would a supervisor from outside the academic world.
Its important to ask for these letters as early as possible, preferably at least 3 months before you need to submit your applications. Professors will be inundated with letterwriting requests at the last minute, increasing the possibility of them writing a poor evaluation. Dont be one of those students.