Okay, not a surprise that waiting tables is an excellent way that newbie actors can earn their rent money while waiting for their starring role. But, the reason waiting tables is so popular is that you often have a flexible schedule that allows you to "trade shifts" with your co-workers should an audition suddenly come up. Additionally, especially in Los Angeles, a number of the more trendy restaurants are hangouts for the Hollywood elite. So, it will give you an opportunity to mingle amongst the in crowd. And by the way, it's extremely rare, but there are still people who get "discovered" by working in a restaurant and coming across the right person at the right time.
Assuming you're of age, bartending is a great way to earn a full time income working part time hours. Many bartenders who work at trendy clubs in Los Angeles and New York have been known to pull down over 100K per year. Seriously.
There are a number of companies in Los Angeles and New York City that work with entertainment companies to place temporary administrative workers. You will typically earn about $15-$20 per hour working as an assistant or secretary. These jobs can be quite valuable in that you may get to work directly (or very close to) some of the bigger decision makers in the fields of film and television. Considering that these are sometimes day, week or month long assignments, they allow you freedom to do the things you need to do with regard to your acting career.
Agencies, film companies and production companies are bombarded with massive amounts of material. The executives running these companies can't possibly read the volume of stuff that crosses their desks, so they hire people to do it for them. This is often referred to as "coverage." It consists of a plot summary of the material and your overall opinion and whether or not you feel it's something the company that hired you should consider pursuing. The amount you can charge for these services depends a lot on what you're reading (script, screenplay, novel, etc.). But the amounts are typically anywhere from $50-$500 -- $50 for a TV script and possibly as much as $500 for a large novel. There are a number of classes and books available on how to write good coverage, so they might be worth it if this sounds like something you might be interested in. Considering you pick your own hours, it's a popular choice.
Hey, it's an ugly job, but believe it or not it can pay the bills. You might even consider role-playing on one of the 1-900 phone companies. It's not the most respectable acting gig, but hey, there's money to be made if you need it.
Lawyers and law firms are constantly in need of people that can provide "delivery" of a court summons. After all, unless a lawyer can "prove" beyond a shadow of a doubt that a summons to appear has been properly served (in person), then their whole case can fall apart. Lawyers pay anywhere from $150-$1500 for this service depending on how difficult they anticipate the service is going to be. Often it's nothing more than simply confirming that the person named on the document is the person you're speaking with and then handing them a copy of the summons and then collecting your check. Again, considering you make your own hours, this might be a road to consider.
If you have a way with words, you might look for opportunities for someone to pay you to write web related content. These jobs aren't as difficult to find as you think. You simply need to do a little research for the opportunity that feels right for you. (Hint: About.com is always looking for new people -- find the "Be a Guide" link anywhere on this page)
Provide a Service:
From dog walker to pet sitting, to personal concierge, there are a number of unique services you can provide that people in New York and Los Angeles would be willing to pay top dollar for. You simply need to find a way to market yourself appropriately and then set up shop. If you do start your own service, be sure to protect yourself with all appropriate insurances, tax filings and by setting up a corporation (so none of your personal assets are at stake should something go awry).
Acting is a difficult skill to master and trying to find work in the field is even more difficult. So, you need to find work that will allow you the time you need to work on your craft as to take advantage of any last minute auditions. The list above has just a few of the options out there. Be sure to visit the forum pages if you have a job idea you would like to share.