From the JetBlue website:
Standby Travel
In order to travel standby, you must have already purchased a seat on JetBlue.
Cities with Multiple Flights a Day
You may list standby at the Airport within 24 hours of your original flight. Standby travel MUST be handled at the Airport and cannot be done over the phone.
You are allowed to travel on the flight prior to your original flight only, regardless of whether it is a non-stop or connecting flight.
You must travel between the same city pairs (specifically, the origin and final destination BlueCity).
This option is free of charge.
If you are booked on the first flight of the day you cannot fly standby on the last flight the day before. However, you may fly standby one flight after your scheduled flight.
Cities with One Flight a Day
You may list standby at the Airport as early as the day prior to your original flight. Standby travel MUST be handled at the Airport and cannot be done over the phone.
You are allowed to travel one day prior to your original flight only.
You must travel between the same city pairs.
This option is free of charge.
Please keep in mind, standby travel is based on seat availability and is not guaranteed. If you purchased an Even More Space on your original flight, we will make every effort to place you in an Even More Space seat; however, it is not guaranteed. The Even More Space fee will not be eligible for a refund if you choose to travel standby.