Personal Identification Numbers (PINs) are widely used for authentication on mobile devices such as smartphones, which act as gateways to many important accounts (e.g., financial, email, etc.). Unfortunately, people tend to choose easy-to-recall PINs involving birthdays, anniversaries, or keypad patterns that are vulnerable to guessing attacks. System-assigned PINs can improve PIN security in this regard; however, they have usability problems such as feeling the need to store the assigned PIN, longer login times, and difficulty remembering. In this paper, we propose, design, and evaluate a set of short training techniques (16–34 seconds) inspired by implicit learning techniques, to improve the usability of system-assigned PINs. We evaluated our designs in a two-session user study with 184 university students. Our results show that some designs offer significant improvements in the login success rate, login times, and user perceptions. These advantages are in addition to our design’s short single-session training, making it more compatible with typical registration workflows than previously proposed multisession training techniques.