PLEASE WEAR A SUIT!
For guys, black/blue suit with a lighter dress shirt (white, grey, blue, etc). Avoid black on black! Wear some CLEAN black dress shoes. And please, no clip on ties. WEAR BLACK SOCKS!!!! Be clean shaven with minimal piercings.
For girls, I'd highly recommend a suit. Skirt or pant suit, is doesn't matter (skirt has to be down/near to the knees). Wear a blouse but don't unbutton your shirt too much. =) Hair tied back if it's long. Minimal jewelry (small stud earrings and a watch should do). And NO OPEN TOED shoes. If wearing a skirt, pantyhouse/stocking is recommended. A small black purse for your belongings should work.
That is basically my "professional" advice.
Seriously, I've seen some horrible attires for interview days. One example is a guy wearing slacks (khaki colored) and a non-matching blazer (olive green) and a dark red dress shirt with a black tie with hiking boots AND WHITE SOCKS! 😡
What is that? You can't afford a new suit cause you're short on money? THREE WORDS: Burlington Coat Factory!!! They have brand new suits for $100-$200 (pants and jacket). In the SF Bay area, they have Clothing Broker's. Best damn store ever. Got my black suit for $110. For shoes, go to Mervyns and get a nice pair for $30. Dress shirt and ties go to Ross for $15 and $10.
I spent $170 and I looked like a million bucks! Apparently, when I lost 30 lbs 2 yrs ago, my old $500 Kenneth Cole suit from Macy's didn't fit me anymore. Funny thing is, I looked sharper in my "cheap suit" than I ever did in my $500 one.