To keep your streak up, you must gain some points every day (between 00:00 and 23:59 in your local time). Your study session has to finish before midnight.
For this feature to work best, you must set your timezone in the settings page. Otherwise, the system will assume you are in London, and this could cause problems if you are somewhere like Australia!
Also, browsing the site/watching videos don't count towards the streak. Anything related to MoveTrainer® (learn/review/overstudy), will count towards the streak.
Please note, that you must *finish* your study session before midnight.