It depends on what kind of paint you use. I never managed to get reeeeeally thin lines with enamels but with acrylics it works fine.
ajlafleche's tip on the shoulder straps should work fine. I once did the unit strap on the sleeve in three passes black-white-black to get a black strip with two fine white lines (never really managed to write "Grossdeutschland" into that srip ![Laugh [(-D]](/FSM/CS/emoticons/icon_smile_newlaugh.gif)
).
For the collar you'll need a very thin and sharp brush (5-0) very thin color, but not to "watery", remove the excess with a cloth or paper from the brush and then draw the line.
The "Liezen" (or something like that) on the German uniform collar shouldn't be that difficult once you get the hang of it, but "SS" runes or ranks really are a pain ... better off with dry transfers, decals or PE parts.