We fly WN and are anal-retentitve first-in-the-A-line flyers, so this is relatively easy to handle. I have a nervous bladder when flying, so hubby sits in the window seat and I take the aisle. When someone sits in the middle seat, I offer to switch with them but add the warning, "I'm sitting on the aisle because I may need to get up a lot. I'd be happy to let you sit here if you won't mind me doing that." Nine times out of ten, they happily take the aisle, and they can't get too upset since I warned them in advance.