Put this in your autoexec.cfg:
Disable shadows
r_stencilbits 0
cg_shadows 0
Simple black blob shadows
r_stencilbits 0
cg_shadows 1
Transparent grey shadows
r_stencilbits 1
cg_shadows 2
Opaque, solid black shadows
r_stencilbits 0
cg_shadows 3
If you would like to change the r_stencilbits cvar in-game, do not forget to do a vid_restart after changing r_stencilbits
Personally, I think the last two options are retarded, since they produce bugged shadows. I would advise you to stick with the blob shadows, since the so called high-quality shadows only look good in stills, not while moving.