1. sell your time 2. (the .5) sell other people's time 3. sell things
I never discount retainers. I've found that clients are more than willing to pay for them and are often happy to do so at a premium. In addition, I charge them the same for strategic consulting as I do for grunt work that their team is unable to perform, such as drawing visio docs for their IT team or teaching them about puppet and auditing their work.
I also retain ownership of work done, generally granting the client an exclusive license in their specific industry, which I spell out in the contract. This enables both of us to establish a competitive advantage. I brand it as a platform, service, or product and offer it to other clients. As I have improvements, it's a good opportunity to reach out to former clients. This type of thing is usually referred to as 'consultant ware' and can have a negative connotation, so I work to make it clean, fast, and pluggable.
The biggest leverage for me is to build my team as soon as I identify opportunities to help the client. I only bring in people who could legitimately be referred to as some of the best in their field. If it's an ecommerce engagement, I bring in people who built much of the product platform. If it's orchestration, I reach out to the inventor and see if he or favored consultants are available.
Here's the twist among other consultancies, however. I do not take a cut of my team's rate. They come through my paper and I adjust my GC, E&O, and other insurance to cover them, but the money I make from them is on admin (for which I bill the standard rate) and through access to other opportunities once I've established my team as a best-of-breed force capable of creating opportunities for the client without distracting their in-house team.
After a year or so, I like to offer the client the opportunity to split my consultants off onto their own paper. We all have great attorneys and accountants, and this can be a risk management technique that demonstrates you're not trying to milk the client through non-value-add services. This is also an opportunity for the consultant to increase their rate, as they will have additional expenses.
As a result, I have never had a consultant turn me down to work on a project. We are fiercely loyal to each other. Do not screw people over when it comes to money. It is never worth it.
Focus on the quality of all deliverables. If you don't have consulting experience with a big 5 agency, work on getting a look at their exhibits. Spend some time creating your brand. Pick (and license if necessary) a group of fonts, create document/email styles and templates, and use them without exception. If you deliver something on paper, use the best paper and printing for the job.
Don't generate invoices by hand. Don't do anything financial on your own. Throw Quickbooks on a virtual machine (their are even hosting services for this) so you can carry it around on an encrypted thunderbolt SSD and update it from anywhere or access it on your private server via RDP. Your accountant will be happy and it will save you thousands of dollars a year. Quickbooks Online and Quickbooks for the Mac are horrible. Don't touch them.
This is what it comes down to: you need to establish a positive reputation as an unflappable authority and you need to guard that reputation jealously. It's what will take you from worrying about having to be concerned about receivables and your next engagement to being confident and able to focus on the client, which will keep money flowing in.
These are some of the things that you'll need to resolve to cross from the ~$300k/year level to the $1-10m/year level. There are additional things to take it further, but they tend to involve less fun stuff than selling, building, delivering, and getting paid.