Not mean... just not that interested works.

If you aren't sure if someone likes you or not you think about it a lot... which makes you think about them a lot which makes you subconsciously develop a thing for them.

So giving off mixed signal... or being "mean" i.e not throwing yourself at someone makes them wonder if you like them or not... making them think about you etc etc

The thing is to find a balance though... too mean is being a cow... too nice is too easy, let them know you like them without saying it, let on just enough so they know they are going to have to try really hard to get you, but they will get you... eventually.