Buy some nice clothes, including a tie.
Buy a book on pre-employment test with practice examples and do the examples.
Know the rules and regs
Know the aeroplane you currently fly
Maybe brush up on your atpl notes if your experience level is low
Go see a movie and read a book and note the newspapers (ie have a life and an interest outside flying).
Know which aeroplanes they fly and destinations
Be ready for the "so, tell us about yourself" question
If you have a question for them, ask it.
Brush your teeth, comb your hair, have photocopies of your license etc handy.
In other words, do the work you would normally do for any job interview.