1. Parents are too involved
Millennials routinely involve their parents in the smallest details of their lives, even after they graduate from college. Recruiters have come to expect heavy parental involvement in the interviewing, recruiting, and negotiation process of recent college grads. Recruiters aren't thrilled about it, but they have adapted to that reality. You'll stand out from the pack as exceptionally mature and professional if you keep your parents' advice entirely behind the scenes; recruiters shouldn't experience any direct involvement by your parents.
2. MySpace mistakes
Do not post anything on publicly accessible websites that you wouldn't feel comfortable showing a recruiter. No racy photographs, for example, or rants about a job or professor you hate. Google yourself and see what comes up, because recruiters will see the same results.
3. Failing to network
Networking is a powerful tool in landing a good job after graduation. You may think you don't know anyone of consequence, but if you sit down and draw up a list of everyone you know (including your friends' parents and your parents' friends), you'll likely be impressed at how wide your network is. Each one of those people has a network in turn, so even with just two degrees of separation, you are well on your way to building a solid network. Let your network know that you're looking for a job, explain what kinds of roles or industries you're most suited for, and make sure to follow through on any leads.
4. A simple "thank you"
When people go out of their way for you in helping you with your job search, make sure to thank them. A short email expressing your gratitude and promising to stay in touch is all you need. Most college grads express poor manners in the job search process, and being polite is just one more way to stand out from the crowd.
5. Bad voicemail greetings
Ninety percent of voicemail greetings I hear when I'm calling recent college grads make a very poor impression. They sound immature and much too casual. Make sure to give your recruiters your cell phone number (so that they can reach you easily), but remember to change your greeting so that they don't hear, "Whatsup, this is Greg, leave a message." A better greeting would be: "Hello, you've reached Greg. Please leave me a message." And if the phone rings at a time or in a place that makes it hard to have an important conversation, let the call go to voicemail. Don't talk to a recruiter during, say, a basketball game.