What is the easiest way to clean a plastic broom (I mean such as the one in the photo) that has collected a lot of floor dust etc?
Just hitting it against a wall to throw out all left overs is not good enough as it is too dirty
You could also comb it, with a hard metal rake. Since you didn't tell us what it is dirty with, this is my best (fits most cases) answer.
Garden hose, or a sink with a sprayer.
You know you're supposed to rap the bristles on the floor at the end of each sweeping motion, right? (not just tapping the armature on something at the end) Otherwise it makes a mess each time you put it back down. It shouldn't ever be all that dirty if you do that.