If you need more guidance than that, all you have to do is casually cruise by the building the day before your interview and see what the people coming out at the end of the workday are wearing. The rule of thumb, though, is that it is better to over-dress than to under-dress. Employers understand that you are attempting to impress them as much as possible, so when you do not put your best "shoe" forward (literally), it can be misinterpreted as a sign that you will fail to put an appropriate amount of effort into the interview.
Also, cruising by the office earlier in the day or the day before can guarantee a good first impression on its own because you will avoid running late on the way to the building and because you will gain enough familiarity with entrances and parking areas to conduct yourself in a manner that does not resemble that of a lost chick asking people, "Are you my mother?"
Punctuality and appearance count for quite a bit when it comes to first impressions. Showing up early is never a crime (as long as you avoid staking out the complex for hours before the interview), and coming prepared can be more impressive than an Italian suit.
If you do arrive a few minutes before the scheduled time, it is appropriate to tell the receptionist or other receiving party your name and that you are aware of your earliness. All other factors aside, this at least allows the employer time to prepare, if necessary. You have arrived slightly prior to the designated time, but you have recognized the ever-so-important point that the interviewer's time is valuable. You do not expect anyone to bend over backward for you; you are simply at the person's disposal when he or she is prepared.
Finally, although it sounds simple, appearing well-groomed and inoffensive makes a huge difference. This is not the day for your experimental hairdo or fashion statement. Being conservative will not make you stand out visually, but it will keep the hiring partner from noticing you in a way that is not conducive to getting a position in the office.
Unless your hair is especially wild-thing-esque or your face is having a particularly unsavory day, it is also best to avoid excessive product use. Too much hairspray, gel, or mousse can create white flakes in your hair that are more noticeable than the strays you were trying to tame—men, this applies to you, as well. Similarly, a caked-on layer of makeup is extremely noticeable when you are close enough to shake someone's hand. This is not a Broadway audition; it is a legal-field interview.
Stay tuned for Part II: Conduct.
