Give yourself plenty of time to:
research the role and the organisation;
think about how well your experience, interests and skills fit the job and the organisation;
research current affairs and trends in your job sector;
find out what the prospective employer is actually looking for;
anticipate questions you might be asked, then prepare answers to these questions;
find out what form the interview will take, e.g. single, panel, group etc.
You should also:
plan the day of the interview, especially your journey with an aim to arrive ten minutes early. Take money in case you need to take a taxi or bus unexpectedly; carry an A-Z street map or put the postcode of the organisation into Google maps on your mobile to prevent getting lost;
decide what you will wear and set it out the night before. Suits and business wear are the best option with comfortable, polished shoes;
get an early night - we all perform better when fully awake.