Sounding Passionate Rather Than Desperate...

Be confident in your abilities. (Exude this confidence the moment you enter the room)

Seem enthusiastic about the problems you solve and the work itself. (Passion for what you do and the things you've accomplished, is a great way to prove you are excited, rather than desperate.)

Be okay with walking away if the opportunity doesn't suit your needs. (Know that you have the ability to say no to an offer.)

Show you are capable. (Prove your qualifications and talents through well crafted stories.)

Be yourself. (Don't bend over backwards to be agreeable or change your position to something that you don't believe is true in order to accommodate them.)

Decide you aren't desperate before you walk in. (Desperation almost has a smell and your interviewers will notice this in your behavior.)