Clear gutters
Brush leaves and other debris out of gutters regularly. This prevents the downpipes from blocking and ensures fresh clean rainwater is flowing into the water butts. Use a brush or pick them out by hand.
Put a wire guard over the point where the pipe joins the gutter to help keep the downpipe clear. The gutter will still need to be cleaned, so collect all the debris and put it on your compost heap or leaf pile.