I take the 8:36 from Crofton Park and very, very rarely get a seat. It is standing room only and can get quite crowded. That said, it's rare that you're packed in to the point of touching. My best advice is get on and move all the way down the carriage. A lot of people get out at E&C, so you shouldn't get stuck and there's usually room at the back. Some carriages are busier than others -- the one that opens right at the steps at Denmark Hill for example.

From time to time for reasons I can't understand they will cut the train from 8 carriages to 4. When that happens, best to get a coffee and wait for the next service.