Keep in contact with them. If they thought so highly of you then, you don't want anything to change that, like dropping off the face of the earth for a year, then reappearing just in time to ask for another favor.
Since they took the trouble to write you great letters, YOU take the trouble to write them thank you notes. They don't need to be long or flowery - just hand-written. And once you know if you got the scholarship or not, let them know. (If they like you, they'll care.) If the scholarship means participating in a special program and you go, tell them what a great experience it was and that you appreciate their help in getting there. Again, not flowery or long - just DO it. That will really set you apart from your classmates.