You don't have to tell her in so many words.
1) Just don't future project with her. In other words, don't tell her anything that would hint a long term or serious relationship and don't talk about things you might do together years from now.
2) Commonly slip in that you hang out with a lot of women and that you see other women. Don't make it sound like you are bragging, but just talk about your daily life and slip in things where you did x with Stacy or y with Andrea.
3) When you start qualifying her, use qualifiers like sexy, fun, crazy, and adventurous instead of things like loving, thoughtful, or other long term relationship type qualities.
4) When you start talking about sex and building Sexual Tension, don't be afraid to talk about what you like and make it obvious that you sleep with women on a semi regular basis. You don't want to come off as a player, but someone who is sexually liberated and quite comfortable with the fact that you see a fair number of women.
5) If she starts getting super emotional, or talking long term, just redirect the conversation by making little jokes and not allowing it to turn serious.
6) When you compliment her, complement the things you are looking for in a sex partner, and not the things that are associated with relationships. If she fishes for a compliment that is deeper, don't give it to her.
7) More than usual, make sure you don't become her psychiatrist or talk about her feelings on anything other than a passing level.
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If you follow those rules, you shouldn't have too many problems with women thinking you want a long term relationship. If the woman immediately latches on to you with the longterm relationship goal in mind, she is almost certainly super needy and you should Eject anyway.