When arriving at the ticket barriers in a station, you should approach the front of the queue, then stop to search for your ticket/oyster card in your stupidly large bag or deep pockets.

You should do this regardless of how much time you spent motionless on the train or going up and down escalators.

It is also quite normal to scrunch up the ticket in said bag/pocket so even when it is found, it will be rejected by the machine.

(Thanks to Wendy for the tip)