Before I go into what could ultimately be an endless exploration of the world of button downs (don't worry, I'll control myself), there's one piece of advice I want to give. I have to admit the fact that though I'm about to endorse everything but the traditional dress shirt, before you can take those leaps, you need the foundation of a white and light blue traditional dress shirt. In reality, you should get two of each, but should you need to be more conservative, this is the only way to go. And as usual, make sure it fits. Shirts come in a lot of different fits, and too tight or too billowy is not the right fit for anyone.
Moving on. Let's start with color. Solid colors will get you very far, especially if your favorite tie is one that doesn't sit well with other patterns. Apart from the blue and white shirts every man should have, stick with solids in agreeable colors, usually ones on the lighter side. You don't want to appear to be cheering on your favorite school.
The last major aspect to consider is the collar. There are a lot of choices out there, but the most useful collar styles will be the straight collar and the semi-spread. Button collars are slightly less formal, but still work well with suits. Stay away from spread collars, unless you're going for an interview on Wall St. Thinner collars are becoming more popular with slim fit suits, but you have to make sure the proportions are right. GQ has a good guide for this relatively new trend here. And if you're really adventurous and going in for a more creative job, take a stab at the club collar, though that might be more for the first day on the job than for the interview.
A few final details: Don't forget about the option of a french cuff shirt. The french cuff shirt can be done well, but in this case make sure you have at least one good set of everyday cufflinks.
The shirt can be the focal point of the outfit because it offers the biggest opportunity to add interest to an ensemble. The range of acceptable dress shirts is vast, but if you find a good fit in a quality fabric, you're set for anything from a fully suited look to a shirt-and-khakis casual Friday. Just make sure you don't take the short sleeved button down dive. Interview or not, that's almost never okay.
Shirts: Banana Republic shirt in Grey, Grapefruit Yellow, Delicate Pink and Light Purple, Paul Smith, 1901, Banana Republic, Calvin Klein