In one word be “neat.” If nothing else someone who is neat and presentable is just as impressive as someone who comes to an interview all decked out. Business casual is the rule of thumb these days. No jeans but a nice pair of pants with a shirt or blouse go a long way. Just make sure to iron out the wrinkles before you leave otherwise you might look like you stumbled out of bed.
The pitfalls of not being presentable … it can be a distraction to the interviewer. I remember a girl that came in for an interview and as I was talking to her I noticed a good sized hole in her sweater and I couldn’t take my eyes off of it. Sort of like when you have a cavity in your tooth and you keep checking to see if it is still there. I really tried to concentrate on what she was saying as I am a big believer in eye contact but it got lost in “did she check her outfit before she left?” I ended up calling her back for a second interview and this time she was dressed nicely and I could now listen effectively and see she would be a good match for the position.
You would think that just your qualifications alone should speak for you but in reality your resume gets you the interview and then it is a combination of skills and presentation. So don’t agonize over what to wear … just be presentable.