Seat belt jams are largely caused by sticky residue that builds up over time inside the belt buckle mechanism. Applying a small amount of lubrication is usually all that's needed to unjam a jammed seat belt. Keeping the belt buckle clean and the metal fastener on the end of the seat belt itself can help prevent any future seat belt jammings.
Spray a small amount of WD-40 or generic lubricant into the opening of the seat belt buckle. Avoid spraying on the seat belt or other interior parts of the car, as the oil will leave a stain.
Let the WD-40 or other oil sit for a few minutes, then press and release the buckle button. The buckle should come undone in a smooth and fluid motion.
Reconnect the buckle with the seat belt so that it locks into place. Pull against the seat belt to make sure the buckle is fastened and secure.
Soak the end of a Q-tip in rubbing alcohol. Rub the Q-tip through the front opening of the buckle and remove any sticky residue that's inside the buckle.