Actually its not the words but the actions that show the sincerity of a guy. Girls mostly go for guys who are sincere and decent, guys shudnt come on strong towards girls. A little bit of flirting is fine but too much is just annoying.
So the tip wod b to be urself, down to earth, decent, sincere and dont try toooo hard!! the person who is made for you will eventually come to u. Let nature do its thing ;)
Approach her when its raining. Go close to her and say "I'm gonna skip all that boring stuff and get to the point, wanna go out with me/I like you"
And remember bad guys get all the girls