This has been an issue with on ear foam pad headphones like the Koss Porta Pros. The headphones are awesome but over time the foam cover wears out due to typical use. Replacements are readily available, but the process of getting them on the headphones tends to stretch them out and then they constantly shift and fall off.
 
When headphones like this ship from the factory, the foam cover feels firm and taught around the edges. They must have gotten that on somehow and I can’t help but feel like there’s a trick to replacing the pads that I haven’t come across yet.
Any tips?