By Margaret Steen, for Yahoo! HotJobs

If you're happy with your current salary, it would be easy to just sit back and enjoy it. But if you want to make sure your earnings keep rising over the long term, you need a strategy to protect against salary plateaus and unemployment.

Experts offer the following tips for maximizing your earnings over the course of your career.

Watch Industry Trends Carefully

"If your professional area is vulnerable to economic shifts, don't cling to it just because it is familiar and comfortable," said Libby Pannwitt, principal of the Work Life Design Group in San Carlos, California. Take a class or even get an advanced degree to arm yourself with skills that are more enduring.

For example, Marianne Adoradio, a recruiter and career counselor in Silicon Valley, said she sees otherwise excellent candidates for human resources positions who don't have global HR experience -- something that is a requirement for more and more positions. She advises people in the field -- even if they aren't currently job hunting -- to make sure they're working on projects with a global component. If they aren't, they need to ask their boss how they can gain this experience.

If You Reach a Salary Plateau, Understand the Reason

In many fields, people start out their careers with a succession of rapid salary increases. These increases taper off after a time, though, unless they enter management. Some companies have career paths for nonmanagers with highly specialized skills, so if you're not interested in management, you may want to pursue one of those.

If the problem is that your field of expertise is no longer in as much demand as it used to be, then you may need to look at a move to a related field.

Make Yourself Marketable Inside -- and Outside -- Your Company

"The most successful people develop themselves to add a lot of value to any company in the industry, not just their company alone," said Steve Levin, principal of Leading Change Consulting & Coaching in Portola Valley, California. This will give you more leverage in internal negotiations -- and more options if you decide to leave your current company.

One tip for being marketable across an industry: Try to work for "name-brand companies," Adoradio said. Recruiters often prefer candidates who have worked for industry-leading companies. Having one on your resume will help your long-term career prospects.

Consider Multiple Income Streams

Some people branch out from their main job to take on consulting work or teach a course in their field. This may help advance your primary career if the work helps you stay current or showcases your expertise. If your second job is in a different field, the second income will increase your earnings and help shield you from the downturns in your main industry.

Don't Focus Too Narrowly on Money

"Raises and promotions are given to people who generate trust and demonstrate competence to handle more complexity," Levin said. Focus on this, and the money will likely follow.

And remember that learning new skills in a job can be just as important as the money.

"When that learning stops, when that development stops, it's time to move," said Leslie G. Griffen, managing partner of Career Management Associates in Overland Park, Kansas.