either that, or just tell her: "I'm not into you. Please stop this."
The most obvious choice is to just tell her that you don't like her like that.
Other options include: None. You want a way that will not make you look like a jerk, right? Be honest.
I would advise against saying anything that could result in getting you the opposite of your desired results.
Example: I dated a girl for 6 months, 5 of those months I was trying to be a distant asshole so that she would dump me. Some girls like the overconfident guys though.
Acting really needy could also backfire on you, some girls feel as though they need that kind of mastery over their lover.
Though if you HAD to lie, you could always tell her that right now isn't the best time in your life to be with anyone. Tell her about a hardship you are going through (If you are lying anyways, you may as well make this one up too.) and reassure her that she is attractive and a good person, either way, nobody likes rejection. She should bounce back quickly and move onto other infatuations. But if you take this approach, don't be caught smooching on another girl.
Bliz seems to have the best advice here. Just tell her you don't wanna date anyone right now.
Hell, maybe she'll say, "I don't wanna date anyone right now either. I just want some hot sex now and then."
Then bust out the hot butter...
Show a picture of the girl you like to her. She'll get the hint.
Or have one of those convos where "It's so good we're friends. You're like a sister to me."
"A sister with whom I do not ever want to see again."
"That is, never see again naked."
"But would quite happily fuck if there were no strings attatched."
EDIT: bastard Jakko, you ruined my joke.
1. say im sorry i only like you as a friend
2. say no
there really isnt any other thing you can do
make sure you say it to thier face, EX - no letter or phone call
dont do anything special for it , EX- dont do it during a date or something
dont beat around the bush be direct with it
be clear with your meaning
be serious, cant stress this enough, if you say it with a sarcastic or jokingly manner she might just ignore the true meaning
be as blunt as possible without hurting her feelings
do not use cliches
DO NOT USE CLICHES ex - its not you, its me or lets just be friends
if she hits you let her unless its for your groin, then youre allowed to block
say youre sorry many times if she starts to cry
if she runs, let her, she may need some time
thats about all i know
kiss her and say it wasn't you.
then you can move on to sex and say it wasn't you.