There are two major things that move you up on the seniority list: new hires starting under you and senior pilots retiring above you. Airline pilots have a mandatory retirement age of 60, so, as long as you are younger than the senior pilots when you are hired, you will rise in seniority over the years. You will rise more quickly if your airline significantly expands and many new hires are placed under you. Seniority is like a game of musical chairs. As long as there is movement (new hires and retirements), folks happily rise in seniority. Sometimes the seniority stagnation can last for decades, depending on the specifics of an airline.
The three most important things in the airline piloting profession are seniority, seniority and seniority. All domestic airlines use a seniority system to dictate your position, which plane you fly (which determines your pay) and what schedules you keep (which determines whether you'll be home weekends, or far from your family, ankle deep in slush during the holidays). For pilots, seniority dictates everything!
In a seniority-based system, all pilots are considered equally qualified, provided they pass the required training and check rides. A seniority system prevents favoritism and other undemocratic practices from interfering with the career of a pilot. The drawback of a seniority system is that even if you're ranked No. 20 out of 5,000 pilots at airline A, with 25 years of seniority and scads of overseas experience in large jets, you will be placed at the bottom of airline B's seniority list if you have to switch airlines for any reason, including your airline going out of business. This means you would likely begin as a reserve flight engineer or co-pilot. Your experience travels with you, should you leave, which might help you get the new job, but your seniority does not. Hence, most pilots stay with an airline rather than lose the seniority they've acquired.