If you notice that your shower or bathtub is backing up, check to see if your toilet is having problems as well.  If so, it is likely your main sewer line and no ammount of Draino will help you.

This kind of advice saves the home owner from wasting time and money messing with Draino and saves the plumber's cable, skin, etc. from being exposed to harsh corrosives.  It's those types of win/win posts that go here.