What’s the best way to “play hard to get”?

Establish that you’re hard to get in general but very enthusiastic about the person you’re with.

Via Richard Wiseman’s excellent book 59 Seconds: Change Your Life in Under a Minute:

As a result of the interviews, the researchers speculated that the best strategy would be to give a potential date the impression that in general you were hard to get (and therefore a scarce resource worth having) but really enthusiastic about him or her specifically. They tested this notion by using some of the same techniques… and found overwhelming evidence to support their hypothesis.