Many job seekers think that whether they land a new position is a matter of luck: it's good luck if they're hired, and it's bad luck if they're not. The truth is that what separates successful job hunters from unsuccessful ones often is a question of preparedness, persistence, and hard work. A little common sense never hurt, either. Below are the biggest mistakes that people make when looking for a new job—and how you can avoid them:

• Leaving your current job before lining up something else. This isn't a good idea even in a thriving job market, but it's an especially bad idea in an uncertain economy. No matter how irritating your co-workers are or how obnoxiously your boss behaves, stick it out until you land something new. Just think of your daily grind as motivation to find a great new gig. The caveats: if something illegal is going on or your office is toxic to your health, get out now and wait tables for a while if you have to.

• Not taking the search seriously. Too many people who say they're looking for a new job take fail to approach it as the serious endeavor that it is. They send out an “okay” resume that they've had for 10 years, don't bother to proofread their cover letter for errors, or flake on sending a thank-you note after an interview. A job hunt is important, and you don't want to burn bridges with potential employers because you're too lazy to put in some effort. The solution? Get serious! Print out your resume and cover letter on high-quality paper, update your resume every few months, and contact the people you plan to list as professional references so they aren't caught off guard when a hiring manager calls them.

• Lying on paper or in an interview. You were just a few credits shy of graduating from college, but that's close enough, right? Wrong. Most of us don't fudge on the big stuff—like fabricating degrees or places of employment—but many job hunters blur the line of truth when it comes to responsibilities they've had or skills they've mastered. Don't risk it. If you don't have the experience you need to land the job you want, work on getting it, rather than making it up.

• Not keeping your network up to date. The worst time to realize you've let your network disappear is when you want (or need!) to look for a new job. Think of your network of contacts, associates, and mentors as a sort of life raft for those unexpected moments that pop up in everyone's career. When you tend to those relationships with periodic phone and email check-ins, coffees, and the occasional lunch, you're maintaining a valuable pipeline that can come in handy when you need to know where the jobs are—fast. If you've let things slide in that area, pick up the phone and ask a few people if they'll have coffee with you. They may be able to help you, but if they can't today, don't make the mistake of letting the relationships lapse again. You never know when you might need their help.

• Not telling the employer what's in it for them. You've got a car payment due in two weeks. You want a better title. You need health insurance. All of these are great reasons to look for a new job, but they aren't great reasons for someone to hire you. See the difference? An employer wants to hear what kind of value you'll bring to the company and why she should take a chance on you, rather than the other 50 candidates. Think about what you bring to the table, and then sell it.

• Leaving the rest up to fate. Your great resume got you an interview, and the interview went well. While you may be tempted to simply wait by the phone until you hear from the company, there's still plenty you can do. First, send a thank-you note to everyone you talked to (within one day of the meeting). Thank them for their time and let them know that you're available if they want to meet with you again. Second, follow up with anyone who may still have influence over whether you get the job. Let your references know they may be getting a call and thank them for agreeing to put in a good word for you. If you have a professional contact within the company, thank them for their help in learning about the opening, securing the interview, etc. Lastly, if you haven't heard from the company in a while, it's okay to place a brief phone call letting them know you're still very interested