I have six rental units which are a good source of passive income. I don’t use a property manager because you are right…they take about 10% of your rent. There are really only 2 things that tenants call about: 1. A plumbing issue 2. Heat/Air problems. Just find a plumber and heat/air guy you can trust. I travel a lot for my primary job. When a call comes in from a tenant, I can call the repair guy in just as easy as a property manager.
Focus on your primary passive income stream first. It's difficult to grow your passive income into multiple streams if you don't already have one, solid stream. Focus all of your efforts on that one project first and help it grow as productive as possible. Then, you can take the money you've made and lessons you've learned from this passive income stream and put them into others.
Nearly every “9 to 5” worker (nothing against them) is a service worker. The definition of a service is a commodity where the value is created and consumed at the same time — think massages, for example. They create value — inputting purchase orders, serving customers, managing employees, etc — for a company or employer. The employer then consumes these services simultaneously.
Using ad networks like adsense doesn’t prevent you from targeting advertisers directly. In fact, that’s how I started; I searched the net for potential advertisers who advertised products relevant to my content and offered them yearly packages. If you choose this option, bear in mind that you’ll need to provide traffic reports and other site statistics.
The age old argument of total return versus income has been, incorrectly imo, categorized as an either or proposition. We are going to do both. Right now I have a lot cash in an on line money market. I also have investments in 2 passive Index funds in a taxable account. We then have substantial 401ks/IRA’s which we won’t touch for at least 10 years. My wife will continue to max out her sep and we will continue to invest in the index funds although with a smaller amount. We have already factored that in. I looked at how to cut into the monthly deficit. Here is what I observed.
Personally, my experience with Amazon thus far has been good - a few of my short eBooks generate steady passive income on weekly basis with zero additional effort from my side. One thing to note though - Amazon pays 70% royalty on each eBook sold only if your eBook is priced between $2.99 to $9.99. Less than or higher than that pricing structure, the royalty decreases to 35%.of your book’s listed price.
So many readers have asked me “How do you invest your money?”.  And so I’ve shared my thoughts on building a smartly diversified portfolio for long term returns.  Of course, this is great when you have a large capital base and 30-40 year time horizon.  For example if you are compounding at just 5-10% but doing it over 40 years and from a large starting base, plus you are topping it up monthly with new funds, you can enjoy ridiculous returns.
Real Estate: I currently own one rental property in San Francisco which I bought in 2003 (2/2 condo), one vacation rental in Squaw Valley, Lake Tahoe (2/2 condo), and my primary residence. Real estate is my favorite asset class to build wealth because it is easy to understand, tangible, provides utility, and rides the way of inflation. I recommend individuals try and get neutral inflation by buying their primary residence as young as possible. The power of inflation is just too hard to counteract.
What I did:The first two years of work in NYC was brutal. I told myself there was no way I could work on Wall St for my entire career because I’d probably die from heart failure by age 40. Having an early death in my mind willed me to save 50%+ from the first year onward and devise a CD, real estate, and stock investment distribution system for my savings every year. I thought about starting this site for at least a year before I hired someone from Craigslist to give set me up and push me forward. Hiring someone to get started is totally worth it if you are a master procrastinator. You can now learn how to start your own site with my step-by-step guide to save yourself time and money. 
Real estate investing falls somewhere in the middle compared to the other strategies we’ve reviewed in the passive income series. The time commitment involved isn’t as strenuous as blogging but is much more continuous than income investing. Start up costs are much higher than blogging or online stores but the benefit of loans gives it more leverage than investing in stocks or bonds.
That depends what you call a “good” lifestyle. Could I rent yachts and get bottle service every weekend living off my passive income? No way. Could I travel 10–15 weeks a year and live in central Tel Aviv with money to spare? Definitely. I was able to earn low 6 figures completely passively. But if you want to make boatloads of money, and step up to the 7-figure territory, then yes, you’re going to need a combination. If, however, you want time to pursue your passions and have a moderate amount of fun, you can definitely sustain yourself with passive income.
If you’ve ever thought to yourself, “I wish there was a product that did this,” then invent it! Create a product, medical or otherwise, and sell it as a company or get royalties for it. It’s not impossible to figure out, I have many friends who have taken a concept to market. Don’t overlook an invention as a fantastic means of attaining passive income.