I go for the ones along the sides in that compartment, between the outer doors and the ones into first-class (assume there are no wheelchair users needing the space) - always avoid the ones immediately outside the bog (would rather stand), but plenty of people seem happy enough to sit there!185 - The fold down seats near the toilets when commuting, if it's at the front end in Sheffield people see the "1" on the door and don't crowd round it as much.
On 333s, I usually go for the single sideways ones between the vestibule screens and the back of the bays of three if I can. These are usually all taken if the train is reasonably loaded.