As of ffmpeg 2.6, we can do even better. BasedBased on High quality GIF with FFmpeg:
palette="/tmp/palette.png"
filters="fps=15,scale=320:-1:flags=lanczos"
ffmpeg -i input.flv -vf "$filters,palettegen" -y $palette
ffmpeg -i input.flv -i $palette -lavfi "$filters [x]; [x][1:v] paletteuse" -y output.gif