If you work on multiple, unrelated projects, one of the biggest potential nightmares you'll see as you scale up is work flowing over into other work, blending together, getting mixed up, and otherwise ruining your ability to think without being stressed out. 

The answer? Build fences.

Clearly separate out different types of work.

Obviously, separating out completely different categories and unrelated projects makes sense, and is even necessary as volume increases. I used to have one email address that all my mail went through. It was easy for a while, but eventually work that was imminently crucially important got mixed in with casual whenever emails, got mixed in with bills to pay, and all sorts of things. I broke things into three discrete inboxes, and that helps. If you have too much email volume on things of varying importance, multiple email addresses by topic -- kept separate -- could be an answer.

Then, letting deals blend together is no good. I just recently created a "Deals" folder on my desktop, with subfolders with everything I'm working on. It's a godsend. All my files are in the right places. I used to do this much more ad hoc, and this is so much better. Actually, I created folders for every major campaign I have... and so far, I haven't had a single thing that doesn't belong in one of them.

Very helpful. Consider doing it.

(My folders: Organization, Health, Company, Writing, Deals, Charity, Money, People, Books)

Even beyond separating out unrelated stuff -- I'd strongly recommend mentally separating and fencing apart different modes of work. In one major company/project I've got, I have three distinctly different hats -- one of them looks like a lot like an executive, one of them looks a lot like a contractor who has fixed spec tasks, and one looks a lot like an employee who should be punching in and out on the clock, getting performance reviews, and otherwise bringing some consistency to some important but repetitive tasks (sales, invoicing, followup, service, etc).

Often, I was blending these areas together, and inevitably one or another ball would get dropped. We just spent some time realizing that, setting some weekly reviews for management, and looking to define out exact requirements for "employee mode." It's a work in progress, but valuable.

Sometimes it's easy to change mental modes, but even when it's easy I think there's some cost to it -- you can't get into a pure flow state. And sometimes, it's not easy, as the different roles in your life compete and fight with each other for attention.

When that happens, build some intelligent fences. Figure out the different modes of work you do, schedule different times for them, and clearly understand what type of work you're doing at the time. Your sanity will surely thank you.