I still find turning off the tight-fisted switch to make strategic investments difficult. Warren Buffet makes it look easy -- and the author's routine prompting is of course superb -- but they remain as poorly-reconciled opposites in my head. Until there's enough cash for delegation and perspective...
I would say: Don't turn off the switch! Not ever.
Maybe a better way to think about it, especially at the beginning: Let's say you get a big order or have a big month and you have just an extra $1000. Then how to spend? Is there a particular ad campaign you'd like to try? Offload some crappy work for 1 month and see how much time it really frees up? Etc.
That is, make it $1k instead of $100k. Maybe more practical?
I would quibble about turning off the switch. It's fine to hand that function to someone else (CFO-type), but when I've had nice budgets (e.g. 5mil) it's been necessary and efficient to ignore the spending noise to some extent and focus on the big picture. But that might be a personal fault.
Take FogBugz, Trac, Basecamp or whatever project management software that you use and spend about 30 minutes to add absolutely every task you can think of to execute on your idea. If you can't find at least 6 months of work then you should probably rethink your idea.
I don't think there's any substitute for experience on this one. The more business decisions I've made, the less I've been concerned that they won't all work out in my favor.
And it's not because I'm playing from a bigger bankroll now. It's because I've learned that losing happens, and it's not the end of the world.