Many people work their way up the ladder in a companies safety department. After 4 years general safety experence you can apply to take your ASP (Associate Safety Professional) test through the BCSP (Board of Certified Safety Professionals). The test is quite difficult however. 4 additional years of experience after getting your ASP you can qualify to take your CSP (Certified Safety Professional) test. With a CSP you will have companys comming to YOU for safety manager positions.
Getting a degree in safety is also helps.