I reached out to several blender manufacturers and professional recipe developers (i.e., people who use their blenders a lot), and the first thing they all stressed is to clean your blender immediately after using it.
To clean a blender, you just need to put a drop of dish soap into the container, add some warm water, cover it or screw on the base, then blend on high for a few moments before rinsing clean.
Sometimes, though, even if you’re cleaning it regularly, it can get a little grubby. “If you blend mineral-rich fruits and vegetables or if you have hard water, over time you may find a white film on the inside of the container,” explains a brand rep from Vitamix. “One way to remove this is by pouring one cup of white vinegar into the container. Then, fill it halfway with warm water and let it soak overnight. In the morning, pour out the contents and use a soft pot scrubber to clean the remaining residue. Good as new!”