Do your homework. It's hard to take a pop quiz when you didn't read the book. Likewise, it's hard to ace an interview when you haven't researched the company. Before interviewing with any company, spend some seriously focused time learning about their business.
Play nicely with others. I cannot stress enough the importance of a team interview. Candidates often think being interviewed by their peers is just a formality. Walking into a team interview with the mentality that the job is already yours will most likely lead to being the last picked on the team.
Do your summer reading. The mind is a terrible thing to waste. Ten weeks of laying in the sun, barbecues, and catching up on your Netflix queue might have you (and your resume) seeming a bit dull. If you've been out of work for a little while, do something constructive that you can talk about when asked what you've been doing since your last role. And no, working on your tan doesn't count.
Don't miss the bus. Punctuality, while not the most exciting word, is one of the most important. I'm not just referring to the obvious of showing up to interviews on time. I'm also referring to the entire interview process. After you interview, be sure to follow up with a thank you letter. If you receive a call, either from a hiring manager or your recruiter, be sure to return it promptly. If you are out of town and unavailable, include that in your auto reply or outgoing phone message, so you don't appear flaky or uninterested. As Thomas Haliburton once said, "Punctuality is the soul of business."