There is and you've mentioned it already: By paying for seat reservations. The only other solution is to have frequent flyer status with BA or any other airline in Oneworld. Bronze on BA (or Oneworld ruby) allows you to select seats for free from 7 days before the departure of the flight. Silver or higher on BA (Or OneWorld Saphire/Emerald) allows you to select seats for free at any time.

PS: It's the same in business on British Airways.