Generally speaking, sit in such a way that you are comfortable.
For the more interested
obsessed advanced learner, you want to sit in a way to present open posture rather than closed posture. This psychologically changes both you and interviewers.
There are a lot of subtle things you can do in how you hold yourself (ie posture) which affect your interviewer. An easy way to put yourself into closed posture is to imagine yourself being cold - you naturally will roll your shoulders forward, pull your arms in, close your legs, maybe cross your arms, lower your chin, and otherwise "close" your posture.
Amy Cuddy has a Ted talk which addresses this exact subject with really good examples. The important take away is that you physiologically change your brain chemistry by your posture. Let me say that again: the way you hold yourself can physically change your brain and how assertive or reactive/stressed you are.
While the whole talk is valuable, the power poses section starting about 10 minutes in is something you should watch. Watch it and cut all the low-power poses from your interview and presenting posture.
Prior to interviewing or something stressful, I deliberately sit in a fairly ridiculously "power posture" way (see the video for good examples) while waiting. It feels silly and completely hokey, but the research on this subject is solid and my experiences fit it perfectly.