I used to keep it short and blunt (in my usual style i guess) and then change the subject.
In the early days when fading it was "taking a break"
Later when I'd quit it was "I've quit"
A life skill worth learning though is that you are under no obligation to answer other peoples questions. As dubs we were programmed to be honest to the point of incriminating ourselves when asked questions by fellow believers especially those with perceived "authority" over us.
"not your business" throws them off guard.
or another way is to answer with a question of your own.
people often feel at pains to be able to give excuses for not doing something they dont want to do, there really is no need at all.