I am a member of UKATTS, I've done really well in the past thanks to the generosity of discounts from various airlines.
But now I have a family in tow I have to pick and choose which offers to take, as most are "standby seats" whereby you only get on if there are seats to spare. If the flight operates several times a day its not so bad, if it's only once a week it can be a problem. Additionally, certain flights to certain destinations are embargoed at specific times of year.
That said, there are many deals to be had via several tour operators, so overall, we do very well.