Elastic and Split Ring
I use an elastic band or hair band and thread it through a hole in the back of the journal, about a qtr to a half inch in from the edge and halfway down from the top of the book. When the elastic goes through I add a tiny split ring that acts as a 'stopper'.
I sometimes colour the rubber bands with Pro-Markers, (although after time rubber bands deteriorate they are easily replaced and in abundance), and also colour the split rings with Tim Holtz alcohol inks. In the last picture I show alternative elastic/hair bands I use. I prefer the split ring to a knot as they look better and lay flatter.