....for my 2c advice with things like rain drops the most important thing is to paint em the colour they 'actually are' rather than the colour you 'know' they are ie clear ...beause they are clear they are showing the colour underneath them and reflecting a bit of what is above ie whiiteish sky colours.
as for lines...for me id start putting in the fine detail in in pencil...but thats just how i work others would approach differently
...hope it helps
"I like to have a thing suggested rather than told in full. When every detail is given, the mind rests satisfied, and the imagination loses the desire to use its own wings."
~Thomas Bailey Aldrich~