none that I know of, besides Ive been on too many Over Sold flights that either left with empty seats or took on standbys to continue trying to play the Game
best advice is to purchase the cheapest tkt you can and HOPE
the day before a long Holiday weekend or after is prime time but this past weekend when I was flying I didnt hear a single request for Vols, I did constantly hear those who Vol Thank You but Please board at your designated slot as we wont be needing any Vols for this flight. I heard this at PHL,SFO,ONT and DEN