When should you show Sexual interest to a Girl?

Well if you keep getting put in the friends zone that maybe means that she is just as nervous as you and she might want you...you just have to go out there and ask her. If you don't want to be just friends there is no risk. You only live once...live life to the fullest and go get the girl!

Wait for the right moment... don't rush into things too soon. if she's sending out hints that she wants you, then that's the perfect time. then again, if the signals are flying that she's just not ready, don't try anything or you'll stay in the friends zone

Well the moment will come, but if that takes too long just give her hints not just straight out telling her, but hinting her that you want to further the relationship.

1) When talking to the girl you are interested use the word "hot" for things about her rather than nice.Or use sexy instead of "pretty"
2) Don't talk about girl stuff..gossips, how her purse looks, what she did to her hair etc...That's what her friends are for.
3) Look at her like you want her, not like your sister. Eyes would reveal much more than you think about your intentions for a girl.
4) I liked what rocket said about "sexual overtones", do that, don't say something perverted though, but afterwards don't crack up, that's for her to smile not you. A small wink would be enough, it will tell her it's funny but it's kinda true, make sure she understands that the joke has some truth in it.
5) Go out with her, alone, and make sure you both order wine, not beer.

If you get put in the friends zone, then there must be a reason for that. She is not thinking of you in a different level or she is not available or she just doesn't give any hints yet. You need to figure out what her intentions are and go with the flow. Sometimes it starts out as friendship and turn into something else, and other times you will find yourself all the way in from the very first minute. So you need to decide after you get the initial feedback from the girl, by her attitudes, by what she says and act accordingly, otherwise you might risk even becoming friends with her. Good luck

Within 2-5 times of hanging out with her, if that's what you really want (ie, you wouldn't want to be friends with her instead and it's not worth the risk). I think girls make up their minds in about 10 seconds, though. So, 10 seconds - 5th hang out should work. :) If you can learn to flirt right then you can test the waters without risking the friendship. Keep it light and funny but with sexual overtones (is that the right word? undertones? - hopefully you know what I mean! :)

I guess there is still more that I can learn from the answers here. Thanks Passenger :)