simple point: If you like a girl. Do not do the 'just friends'
method and hope to progress from there. Girls will too much of a
shock if u suddenly change direction with her.
If you want, right from the start, show interest more than just
friends. You've got to be obvious, so at least it prepares the
girl. Once her perception of you has been cemented, that's it. Very
hard to change unless due to extenuating environmental
factors.
Go read up on the ladder theory about men & women 
