I just turn my suitcase onto its long side (without the wheels) and place it on the floor at the end of the bench, making sure that it isn't partially obstructing a door. Since you get on at the start of the line, you can usually snag a seat at the end the bench - avoid silver seats which you may have to give up for customers who need them.

Here in London, I sometimes help the grockles on the Piccadilly Line by suggesting that they put lay their suitcases on their sides in the luggage area. But some of them still choose to keep enormous cases, wheels to the ground, in front of themselves. At best they block the narrow aisles, and at worst they roll into the knees of other passengers.