Get headshots taken of yourself. Because you are only seeking extra work, this doesn't have to be a huge expenditure. Some photographers will charge a few thousand dollars for just one session. For your purposes, find a photography student or someone who will just charge a couple hundred bucks and let you keep the negatives (or digital images) to make copies.
Select an array of headshots that represent exactly what you look like on film. When you get hired, they expect the person in the picture to walk through the door. This means if you're 60 years old, don't use a headshot from 20 years ago. It's okay to look like yourself--casting directors prefer it that way. Also, make sure you have different "looks." This means a shot of business attire, dressed casually, dressed funky. Depending on what the movie is, all of those shots will serve different purposes.
Find an extras casting company in your area. Film production is no longer just in New York, Chicago and Los Angeles. Almost every state has offered a tax incentive program to bring productions to them. Therefore in just about every state at least one (if not many more) movies will shoot there each year. Find the companies in your area and submit your headshots.
Keep your contact information current. When a big budget movie is shooting and the scene needs 200 extras tomorrow, that extras casting company is going to call twice as many people. The first 200 who return the call will get the gig.
Try to become a SAG member. SAG stands for the Screen Actors Guild and is the professional union for actors in movies and TV. (There's a sister union AFTRA which does soap operas and a handful of primetime shows.) Every SAG production must have a certain amount of extras who are SAG during the course of the production. If you earn a certain amount of money, you will have health benefits and a pension after 5 years. This is ideal for the older person who wants to get out of the house, make some money and get some benefits.
Consider incorporating yourself as an actor. Most people who do extra work also have another job. As an actor, you are allowed to become your own business. This opens up a whole additional list of deductions available to you from the IRS. It costs a few hundred bucks (each state is different) but if you start making upwards of $20,000 per year, it will be well worth your investment.