While you may not be able to create passive income right off the bat with your service business, there are some very quick ways that you can start the process of automation. Some of these tips are dependent on the type of business you are in, but most are universal.
Create standard operating procedures
As your virtual assistants are doing work, it's super important that you have them document the steps they are taking. Of course, if they are just going off of the training you originally created you don't need to do this, but when they venture into new areas make this a priority. This will save you tons of time retraining, and will also provide your other VAs with information should they need to step in.
Your standard operating procedures documentation doesn;t need to be anything fancy. In fact, if could be just a folder full of Google docs or spreadsheets. You need to make it easy enough for your VA to do on a daily or weekly basis, but detailed enough for someone brand new to be able to implement it.
Train, train, train. . .
Make sure that your guys know the ins and outs of the job. If you train your staff properly, they will be able to go a few days without your direct intervention or feedback.
When you take the time up front to train them well, you'll reap the benefits later in being able to let them loose on a project and they crush it every time.
Find joint ventures
If you are a web developer, find a designer. If you are a marketer, find a sales guru. Whatever you do, you can always find someone who you can work with that will allow you to become the middle man.
While it may seem like your partner gets the raw end of the deal, you need to realize that you are training a staff, building a team of professionals, and executing.
Personally, because my full-time job hurts my ability to sell throughout the day, I've teamed up with a couple of local entrepreneurs that do it for me while I, and my staff, provide amazing web development and products.
Set a time for regular reporting and feedback
Make your VAs report on what they've accomplished, and provide timely feedback (I'm horrible at this, I admit). As they report and receive feedback, their start to hone their skills in whatever it is that they are doing for you.
After a while, they'll start to see the big picture, and will offer suggestions and so on. This is great, because they'll become your extra eyes and ears, and will provide more value to you, which you can then give to your customer (along with a price increase).