I learned this from The Fly Lady (I find her writing style annoying but her methods work): get a cylindrical toilet brush holder that can hold water for each bathroom. Fill half way and add a squirt of cheap shampoo (Apple or Peach Suave is nice). Swish the bowls every morning. Also keep an all purpose cleaning spray in each bathroom and a roll of paper towels (or a stack of washcloths) and wipe the mirror, sink & countertop after you get ready in the morning. She calls this a Swish & Wipe. If you do it everyday your bathrooms will always be guest ready.
Consistency is key and it really works. It is a great habit to instill in kids too.