Making a good first impression is vital for anyone going for an interview. Us humans can't help ourselves, we have to scan the person in front of us to make a judgement of whether they are a threat to us, or not (in a primal way) and in a non primal way we look to see if the person is fitting in with the standards of our workplace, particularly if we are interviewing them. If you dress to impress, and I mean give the impression that you look like the 'type' of person the company hires, you will start on the right foot. If you go in with an outfit that is not of their environment you will stand out, for the wrong reasons. Do check with the HR team, particularly if the company is a casual wear company, such as a technology company, what they expect you to wear to interview, don't assume they will be happy for you to turn up in casual wear. When I worked i recruitment at Intel most of us wore hoodies and jeans yet for some reason, not my decision I might add, we wanted candidates to turn up in suits, it was a strange experience interviewing candidates in my jeans and trainers while they had their best suit on ( this rule may have changed since I left though).
I once interview prepped a candidate directly before they went for an interview at a legal firm and noticed they had quite dirty nails and hands and had to show them the way to the toilets to give them a scrub, these are the tiniest things I know but they can't be missed. Too much perfume, or aftershave, can also be a problem, I recommend going natural with just regular deodorant on, you can't predict what memories certain smells evoke in your interviewers minds and you wouldn't want it to evoke a bad memory, or worse set off an allergy!
If you need further help I do cover this in my training and can give further helpful tips and direction on what to wear.
Here is another good article on dressing to impress http://www.inc.com/quora/7-smart-ways-to-be-amazingly-professional-and-look-the-part.html
Clare Reed is a leading global expert Interview Coach with over 24 years global interviewing and coaching experience.