use the poly lasso, just zoom in and keep clicking and clicking and you will end up with a smooth line. If you space the anchor points out on a curve it will end up looking jagged.
there's a magnetic one,too, but when i use it, say to just go round a head lamp,it some times sticks and the lasso puts a line i place i dont want it to. how can i stop this?
if you do use the magnetic id recomend you click the mouse around places where the line your trying to trace is real faint other wise your line will jump all over the place. I prefer the poly but whatever works for you.