I hate cleaning out paintbrushes so I tend to buy cheap ones and throw them out after use. However, the bristles tend to come out and I have to fiddle about getting them off the surface I'm painting. Good brushes should last years so it's worth looking after them. Water-based paints can be washed out in water, but for oil-based paints use Polycell Brush Cleaner.
You will need
Step by step process
Pour the cleaner into a used glass jam jar or similar receptacle.
Work the brush up and down in the liquid.
Then simply rinse with water. The cleaner can be used several times, which makes it particularly good value for money.