Dress
If you are applying for a conservative company dress accordingly. Men should opt for a dark suit, plain shirt, tie and good shoes. Women can go for a business like skirt or trouser suit. A calm hairstyle and discreet jewellery and make-up are essential.However, if the company is a little more relaxed you can add varying degrees of colour. Still look smart and organised, because no matter how laid back the company may appear, you still have to make a good first impression as someone who will give 100% for the sake of the company.
Don't wear jeans, wrinkled/dirty clothes, anything that is in style (e.g. hair bandana or giant hoop earrings) and take out any pierced jewellery that wasn't intended for the earlobes.
If the job you are applying for is summer work, then the best thing to do is dress smart casual; chinos, khakis, shirt and good shoes.