I pick up any mess that I consider to be "outside of the norm" which is things on the floor etc. I do generally try to pick up food and put it all on plates and tidy up a little to make it easier for the serving staff to clean up the table. I also clean up any food that just gets onto the tables and put that onto plates when we are finished too. I'm not a naturally neat person (I wish that I was), but I just don't want to make serving staff member's jobs more difficult than they already are. Although, if they are giving crap service then I'm not so eager to clean up mind you.