Having worked in and around the airline industry for about 30 years, (and my father before me) I can say with confidence that there is no set rhyme or rhythm to how airfares are set. Mostly it is what the market will bear. Airlines compare many factors. Actual costs, load factor (% of seats filled to seats available), total demand, # of flights offered by competition, and of course price charged by competing airlines.
Some instant sales will occur if a large percentage of unsold seats are predicted from a certain city pair. i.e. MIA-SJO. So it is just best to check often and be ready to buy.
And remember that fares usually fall during the off season for travel.
Good luck, have fun and please make sure you seatbacks are in the upright and locked position for takeoff.
