change \itemsep vertical distance in lists

[Robert Neumann]

hello,
is there a way to change the vertical space between the items of a list? I 
tried to use an ERT with \setlength{\itemsep}{0ex plus0.2ex} but somehow it 
does not work. I would like to change the topsep (distance to the preceeding 
text) as well, I'm using lyx 1.34.
Thanks
Robert