i'm gonna stick my neck out here

seriously all ment well > i think you should first try to use more c&p parts and really master the merging and stuff , then try to re-brush small parts and gradualyy build up like this ..
also really watch the selections ..
when you are using the polygonal selection tool you can see a box above .. feather ..
in the box there is a number .. ususally it's sais 0 (zero) ..
for more soft selections yo can set that to different modes..
for instance 0,2 ... then the selection will be a tad more soft and less choppy ...
it's not most important because even on feather 0 you can make great selections without choppyness , but usually on lines with are not straight it's best to set to 0,2 to get more soft lines but still have a sharp selection
try out and set it to feather 1 , you will see a very blurry effect , this is very usefull for highlights and reflection work ..
but for all the main lines never use anything above feather 0,2 or max 0,5 (depending what your doing)
also try out if feather 0 is good enough
just try out the settings for instance feather 3 can be great for shaded or real unsharp ghost like stripes / reflections
also really zoom in when selecting so you can just see what you are doing but really see the diferense between the line and other parts
maybe you already knew this but maybe you did not .......if not it's very usefull
