Are we talking the front seatbelts here? As far as I'm aware the interior trim panel should just prise out of it's location. You should then be able to apply lubricant to the mechanism.
Alternatively I believe someone suggested trying furniture polish. Pull out the seatbelt to it's fullest extension and apply polish to both sides. Allow the belt to coil back and then pull out and re-apply if needed. The polish should help lubricate the mechanism.
