Ideally, you will want to avoid having to answer this question for a while (definitely until after sex), but eventually, this question or a similar variant will come up: "What are you looking for?"
If you want a relationship
If you want a relationship, this question is easy to answer. You want to make it clear you are open to a long term relationship, but don't do it in a desperate manner. Think something along the lines of: "I'm pretty open but I would be happy finding someone I can have a real relationship with." Most women, even if they do not want a relationship with you, are probably going to be ok hearing this. A women will likely be much more turned on by you if you consider them potential relationship material versus nothing more than a piece of meat, even if she wants to be your piece of meat and not much more!
If you are just looking for a quick lay
Herein lies the difficulty of this question, provided you do not want to blatantly lie. With that said, if you were given one get of jail free/lie card, I would use it here. Similar to the point above, even if a woman is just looking for a friends w/benefits type arrangement, the last thing she wants to hear if she asks you this question is that all you want is a hole to stick your dick in. A woman who wants to be intimate with you, even if not in a relationship context, still wants to be romanced and turned on. Telling her you are just looking for a slam piece isn't going to help your cause.
Now what do you do if you do not want to blatantly lie? The solution is to be as vague as possible. I generally say that "I am pretty open to whatever. I generally go with the flow and see how we both feel." Another approach is to use the amplification strategy I discussed in a similar article on how to deal with the: "Are you seeing someone else"
question. Amplification means answer via ridiculous exaggeration. For instance: "I am looking to get married in the next two months and having 12 children, minimum." or "I'm looking to break the hearts of innocent women I meet at [wherever you met]". Use this amplification/humor approach to evade.
If you aren't sure
If you aren't sure, then the answer is also relatively straight forward: tell the truth. Say you don't really know what you are looking for. This is similar to the "quick lay" answer only you can feel better about it because you are not lying.
Cost-Benefit Analysis of the Lie
I don't often suggest lying when it comes to dating and relationships. However, if a woman decides to ask you this question (which I don't think is a good strategy for women to ask anyways but they do it), she is basically forcing you to answer a particular way or face pretty dire consequences if you are looking for a quick lay and are honest about it (high chance of failure to get what you want). You need to decide what is most important to you: you can either give up on a high percentage of women who ask you this question (if you wouldn't consider dating them) or lie. The choice is yours.