I just use quvi for that.

Example:

    quvi -vm --format $format "$url" --exec 'wget %u -O %t.%e'

instead of $format put any format that the video supports. To query them, use:

    quvi --query-formats "$url"

So it's going to be something like:

    quvi -vm --format fmt43_360p "$url" --exec 'wget %u -O %t.%e'

And to extract audio from the result you can use:

    avconv -i something.webm -vn -acodec copy something.ogg

Youtube however is switching away from fixed video files to separate streams to be used with MSE. You can note that higher resolution video is not available the old way. So downloading that won't be so straightforward.