I find this one really tricky, too! I tend to say to people "I'm getting off soon, would you like my seat?" - tends to turn the trick. However, I've not worked out how to stop people glaring when my knee has given way (happens every couple of months or so), and I *can't* stand for the 15 minute rail journey. I just have to (slightly) exaggerate the limp as I get off the train in the hope that they will feel guilty for their glares!