Plan your journey
Unless you're one hell of a persuading talker, you're unlikely to get the job if you turn up looking like a big sweaty mess, so it's absolutely integral to plan your journey beforehand. If you're going by car, try driving past the location in advance so that you know the route and (if you really want to be prepared for any possible scenario) seek out an alternative route there.
Similarly, if you're going by train or bus, then check the departure and arrival times in advance and keep an eye on any maintenance work on the lines or anything else that could cause a delay.
Finally, if you're lucky enough to be able to walk to your job interview, ensure that you know exactly where the office is and that you can gain access to the building - you don't want to be stuck outside while your potential new boss thinks you just haven't turned up.
It's not always easy to sleep the night before a job interview, and that's why it's important to give yourself the best chance of getting a good night's sleep, so go to bed relatively early and set your alarm. If you really can't sleep though, just have a coffee or two in the morning - after all, you've only got to be alert for the short duration of an interview.
Read the news
It may not seem like the most obvious part of preparation, but it's always worth reading the news on the morning of a job interview. Many employers like to see that potential recruits are aware of the goings on both in their industry and in the wider economy, so don't be surprised if they throw in a question about what impact you think X will have on their business.
If you're unsure of the dress code then always dress smartly. Turning up in just trousers and a shirt can give the impression that your attention to detail might be lacking, so get your suit ready and ironed the night before. There's no need to buy anything new (unless your wardrobes consists entirely of band t-shirts or football tops), but try to wear something that will make you look smart but also feel comfortable.
Men should remember to shave, and women should refrain from wearing any strong fragrances or too much make-up and jewellery, while smokers should attempt to resist the urge to light up immediately before the job interview.
Finally, it's a good idea to bring with you your CV and a portfolio of your previous work, if appropriate.
The employer will almost certainly have your CV and application form to hand, but taking a copy with you shows that you have considered all possibilities. Likewise, the interviewer may not have asked to see any of your past work, but bringing along a fewer relevant examples is unlikely to hurt your chances.
Also, take along your notes so that you can do some last minute revision, as well as a bottle of water and some cash for a last-minute taxi.