Create your own agency
Cliff Ravenscraft, who runs The Virtual Assistant Podcast, uses an American virtual assistant company called Contemporary VA. The company itself was founded by a virtual assistant that decided to start her own agency. She finds talented people from around the United States and pairs them up with business owners that need help, but don't necessarily want to hire a full-time w-2 employee.
Hiring VAs to be dedicated support to clients is a great way to build a business that is semi-passive. Most of the time spent would be in support/training of your VAs. As things continue, you'll see the need to intervene slowly decrease.
Create packages for your service
Once you've trained your staff, you can start to create packages that you can easily sell to clients. For example, with Internet marketing, I have a package that contains article marketing, another that has blog posts, and nother for video marketing, etc. One you have those, clients can sign up to your service and you just give your trained VA guidance on what to do based on the package and let them go at it! You may need to review every now and then, but ultimately it's pretty simple.
The more you train your staff, the more independent they become; the more independent your staff becomes, the less you need to work.
Seek out retainers
After a while, you can offer retainers to your clients, in which they will pay for the availability of you or your staff. This is great because it's stable money, and it is not time dependent. Of course, the terms of the retainer are dependent on you, but you can figure that out. For example, if you are a web developer, you may have a retainer that states that you'll make yourself available to work maintenance or start a project.
Keep in mind, retainer doesn't necessarily mean that you won't charge for the actual project, but that you'll make yourself available.
Action steps: create passive income ideas for your service
As you become better at delivering an amazing product people will start demanding your time or your VA's more. How are you going to use this to your advantage? Just as with a product, you want to create passive income. Create some scenarios for creating passive income for your service and leave them in the comments.