Sometime during a job search, many people ask me whether they should consider a change of career. Before I answer, I always ask, "Why are you considering a change?" Their answer tends to be one of three: They can see that their old line of work is disappearing; they’re no longer happy in a profession that once seemed great; or they need something their current job can’t provide – more money, more opportunity, a chance to travel or have greater control over their time.

I ask because the answer will help them get clear on both whether they should make a change and how to do it.

You’re going to change jobs anyway: In 1955, the average person stayed in a job for 27 years. Today the number is closer to 4 years. America’s incredibly dynamic economy is constantly building new business, and constantly destroying old businesses. Like it or not, change is going to happen, and the question really is, will you plan the change or will you be forced into it?

If you want to change careers, you’ll have to persuade your next employer that you can do the job. There are at least three paths toward switching career fields:

The Training Route:

If you are unemployed and getting professional job-searching advice in government or private career counseling services, you’re already hearing about training programs, and many of them are very good, but you have to be strategic about what training will prepare you for the next job. That means looking ahead to the skills required in the jobs you might apply for later.

Check out the "Required Skills" list in job advertisements. Computer skills are obvious choices, as are vocational training in work that requires certification, such as health care, financial and transportation work that requires a special license.

The Two-Step Route:

Big career leaps can be made in two steps. First change employers, then fields. Here’s how it works: Say you’re an accounts payable representative and think you’d good in public relations. Find out as much as you can about the public relations business using online resources. Go for informational interviews at companies with strong public relations departments (or better, full time public relations, marketing and advertising firms).

Once you’re in, learn about the business, the skills you’ll need, and the path to advancement. It’s best to be upfront from the beginning at the new firm, because if you do your accounts payable job well, but also talk, train, and observe, you can be considered for a public relations job when it opens up in the firm.

In the coming years, successful companies will pay more attention to hanging on to their best employees, even as they switch careers. You can also reverse this method, by moving to a new department in your current company, then switching employers. Note, however, that your current employer will want you to stay for a while in the new role.

The Volunteer Route:

Another way people get the experience they need to make a case for changing careers is to prove their skills outside the workplace. They can do this in a volunteer or service job. Say you want to prove your potential in public relations to an employer – if you’re part of a town group raising money for a new library, or a church committee, volunteer to publicize the cause, to reach out to potential donors in email, with publicity campaigns, working with the local newspaper or radio stations, and handling all the other activities required.

Keep a close diary of what you do and the positive results. Then build success stories around those activities. Be as specific as possible, and get others to recommend your work. You might not be paid for volunteer work, but you can build out your track record of skills in the job you want to have.

Changing jobs and even career paths is pretty much inevitable in the 21st century. The only questions are whether, and when, you’ll be in control of that change.