I never had much luck keeping zinnias alive in the ground, but this year I put some in a window box and they've done fairly well throughout the summer. But now they're getting scraggly and leggy and falling over with the lower leaves turning brown/falling off. I'm not sure what to do with them at this point (see pic).

I know they're annuals, so should I just consider them something to be enjoyed for a season and then discarded in the fall whenever they stop making pretty blooms? Or is there something I can do to perk them up and maintain them?